As I remember well, Killrob said it won't be a automation SIMULATION game, but a TYCOON game. Going so deeply in-depth onto wheel lock and such, is overkill as its a tycoon game. Engine compartment space might be later on actually, as you choose your body you might be dealing with a min. and max. engine size. Thinking about big pickuptrucks, not having a real limit, but small compact cars not being able to have more than 1.4 or 1.6 engines. Not too sure about this, but it sounds reasonable to me that you can't make a Smart with a 5.0L V16 engine it it of course.
As you are referring to flip,move,remove, retract and hide for bodyparts, like hardtop/targe, it is actually quite simple. This is a gamedesign explanation, but i'll try to keep it simple:
The body will be made and any removeable or additional part of the car body can be added as some sort of second or even third item to the body. By having like a menu that says: Targe YES/NO and you select No, you can make that part disappear and get the body to be called Targa quite easily by using code programmed in. It's not all that hard, but it is quite some work getting it to work correctly at first.
After all, it's hard to create a lightbulb for the first time, but easy to reproduce it
