First , keep doing orders...once you had enough level , the parking , junkyard , barn ,auction will be open....than you can used the money you have to go most of those place(beside parking....that's where you store your cars) and working on it.

Second , you can only insert the engines that was in the car's engine swap list....by pull out the old one and put in another one that you either pull out form some other car or built on the rack.

All of this features will be bit by bit unlocked when you grain more and more level , and upgrade the garage in toolbox with those points.