Go to the game directory. From there:
Forza Motorsport/media/pcfamily/renderscenarios

From there, select the folder which corresponds to your ingame Dynamic Render Quality or whatever it's called (in my case, i use ultra).

In the folder you choose, open global.xml with notepad. Before proceeding, I'd recommend making a backup so you can revert if anything goes wrong.
In global.xml, once open hit CTRL + F, and search "CarDamage". Change this from 1 to 0.
You can also search "CarDamageTextures" and do the same.


You'll still get cracks in the glass, but now your car will no longer get scratches or dents.