AS THIS IS ABOUT SAVEGAMES MAKE SURE TO MAKE BACKUPS BEFORHAND IN CASE SOMETHING GOES WRONG!
I WON'T/CAN'T BE HELD RESPONSIBLE FOR ANY KIND OF SAVEGAME LOSS REGARDING REINCARNATION AND/OR MAX DAMAGE!
IT WORKED FOR ME BUT IT MAY OR MAY NOT WORK FOR YOU!
USE THIS WORKAROUND AT YOUR OWN RISK!
YOU HAVE BEEN WARNED!

Also if you find any spelling or especially grammar mistakes (there will be plenty I'm sure of it) keep them. I dont need them.

Broken Achievements:

• Slap It in 'Er Slot (Buy your first Part from the Parts Shop)
  
• Filled 'Er Up! (Buy and Equip 12 Parts in the Parts Shop for one Car)
  
• 50 Shades of Red (Complete 50 different Career Events on any Difficulty)
  
• Truly a Stunning... (Achieve 100 Cunning Stunts)

Challenges you can complete but the skins and rims wont unlock:

• Hotwiring Hotstuff (Unlock 26 (27) vehicles.)
  Broken Unlocks: Eagle R Skin 'Wild Cat', Crazor Rims 'Spurs'
  
• Car Tart (Drive 26 (27) different cars.)
  Broken Unlocks: Volkswerker Skin 'Completely Buggered', Siamese Dream Rims 'Aero MK2'
  
• Split Personality (Split 50 peds in two.)
  Broken Unlocks: Countslash Skin 'S&M', Volkswerker Rims 'Dino'
  
• They don't like it up 'em (Impale 50 peds.)
  Broken Unlocks: Crazor Skin 'WT Actual F', The Bear Rims 'Korsakov'

Challenges you cant complete:

• Pis Taker (Purchase 85 different PowerUps)
  Broken Unlocks: Towmeister Skin 'Sindy 500', Snaky B*st*rd Rims 'Prang'

So far I managed to unlock ALL the broken skins and rims aswell as the 'Truly A Stunning' achievement using the workaround.

The '50 Shades of Red' achievement can be unlocked within Reincarnation.
Thanks to TheOdds for figuring it out.

The Upgrade achievements still need confirmations. I cant test them since i already had those. Feedback is welcomed.

This workaround is for unlocking the 'Truly A Stunning' achievement and fixing the broken skin and rim unlocks.

Short Version:

The actual workaround is pretty simple. You basically take your Reincarnation save, copy it into the Max Damge save folder, complete the tasks required and copy it back to your Reincarnation save folder.
There is one thing I need to mention tho. Your carrer progress will drop down to 93%. Reason for this are the new Classic Carma maps in tier 03 & 15 in MD. So you have to finish these maps again in Reincarnation (in all 3 ways) to get 100% again. The cop cars stay unlocked tho

Step-by-Step-Guide (aka Long Version):

• Reincarnation savegame preparation
  This is necessary so you get the achievements that are tied to the challenges. So basially finish the challenges first to get the achievements and then we fix the broken unlocks.
  
  • Finish the 'Hotwiring Hotstuff' challenge.
    
  • Finish the 'Car Tart' challenge.
    
  • Finish the 'Split Personality' challenge.
    
  • Finish the 'They don't like it up 'em' challenge.
    This ones a bit tricky in Reincarnation, but doable. You neeed to pierce the body of the peds and get a 'Skewered' or 'Kebabed' Bonus. Look for sitting peds or penguins, they are the easiest to impale.
• Locate your Reincarnation savegame.
  (...\Steam\userdata\%YourUserAccount%\249380\remote)
  
• Make a backup of it.
  
• Locate you Max Damage savegame.
  (...\Steam\userdata\%YourUserAccount%\505170\remote)
  
• Make a backup of it too.
  
• Copy over your reincarnation save to the Max Damage folder.
  
• Overwrite the existing save (You did make a backup, did you?)
  
• Complete the tasks that are required to unlock the achievemnts/skins/rims.
  
  • 100 Cunning Stunts.
    They are easy to get, basically any jump that includes a flip or roll and you ending up on the wheels without using (whobble) recover counts as cunning stunt.
    NOTE: MD needs only 60 stunts for the achievement, but Reincarnation requires 100.
    
  • Finish the Hotwiring Hotstuff challenge in Max Damge.
    In MD you need to unlock 31 vehicles for this challenge. We already have 26(27) from Reincarnation + the Iron Hawk + the TEZ Eagle for non KS-Backer. So you only need to unlock 3 more of the remaining 5.
    
  • Finish the Car Tart challenge in Max Damge.
    Simply drive the new acquired vehicles.
    
  • Finish the Pis Taker challenge in Max Damage.
    This is an expensive one. You need about 2mil credits for all PUps, but luckily theres that credit cow in chapter 15.
    
  • Since we did all the required ped splitting and impaling in the preparation theres nothing to do for these unlocks here.
• Once the tasks are completed copy the savegame back to your Reincarnation save folder (You did make a backup of the old one too, did you?)
  
• Start Reincarnation. The 'Truly a Stunning...' achievement should pop up and all the skins and rims should be properly unlocked now.

NOTE: I recommend doing it task by task. Like do the cunning stunts, check if it works, do the Hotwiring Hotstuff challenge, check if it works, and so on. That way you dont have to redo everything if something goes wrong.

Alternative:

You can simply copy your Max Damage savegame into the Reincarnation save folder. However the achievements which are tied to challenges wont unlock that way if you didnt got them already.

• Slap It in 'Er Slot (Buy your first Part from the Parts Shop)
  
• Filled 'Er Up! (Buy and Equip 12 Parts in the Parts Shop for one Car)
  
  Those achievements wont unlock if you let your cars upgrade automatically.
  So delete (make a backup) or move you savegame to somewhere else and start a completely new one. then upgrade your car MANUALLY.
  
  NOTE: Unconfirmed. Need feedback if this actually works.
  
  
  
• 50 Shades of Red (Complete 50 different Career Events on any Difficulty)
  
  Apparently Reincarnation only counts all non-Classic Carma events towards this achievement. So once you finished all events you only have 34 instead of 50 (the missing 16 are the Classic Carma events that did not get counted).
  Luckily the statistics in the savefile are cumulative between different saveslots. So all you have to do is starting a new carreer in a new saveslot and do 16 more non-Classic Carma events and the achievement will pop.
  
  NOTE: Confirmed. It works.
  Thanks to TheOdds for figuring it out.

How do I check my stats?

In the carreer window go to the My Stats page (default key: X) and under 'Criminal Record' you find all the various stats. All but the amount of how many cunning stunts you already have done.

How do I check how many cunning stunts I already have then?

You have to check that directly in the savegame.
Go to the savegame folder and open the lua-file with any text editor (Notepad, Edit+,...)
Search 'cunning' and you see how many stunts you alredy have.

Then why not editing the savegame directly instead of doing this workaround?

It doesnt work. I tried editing the savegame using various methods. Text editors, HEX editors, even the statistic tab within SAM. While you indeed can simply edit the stats, they either wont get carried over (SAM Statistics) or the contents of the savegame got erased when you load the savegame inside Reincarnation/MD. There is somekind of legitimacy check of the savegame and I dont know by what and how to bypass it.

Can I unlock other achievements/challenges using this workaround?

Yes you can.
Basically all achievements that are not tied to any challenge should be able to unlock using the workaround. Achievements like 'A Ton Of Tin!', 'A Load Of Laps!' and similar are unlockable this way.
However achievements that are tied to challenges wont unlock. Basically all of the 'Complete the [...] challenge' achievements.
At the very least you can bypass the broken stat counts for the achievements tho.
As an example the stat count of the Deathrace lap achievement was broken for me too. It only counted the 1st lap, but not the rest. So I did all the races with my Reincarnation save in MD, copied it back, did the last race in Reincarnation and got the 'A Load of Laps! achievement.

Credit goes to TheOdds for figuring out what is counted for the '50 Shades Of Red' achievement.and therefor finding a solution to unlock it.

2016-29-11 @ 12:22 pm

• added 'Additional Notes' section
  
• added 'Version History' section

2016-28-11 @ 11:29 pm

• changed/rephrased 'The Workaround' section

2016-28-11 @ 03:22 pm

• Initial post