Hi, I'm making something similar.
Player opens a menu to setup his car, when all done, on exit I build all parts in a string, every part is defined in a bool (is it on or off), after that I have a string 010110101010 for example, where every 0 or 1 means true or false. When car is spawned it checks which part to add.
I'm doing this way so I can send that string to others in multiplayer too, to spawn the same car+parts.
Maybe you can use this method. Depends on your game setup, if lots of parts that could be hard to manage 0101010100101010100101… etc.