Don't know just how old you are but if you're real careful you can melt a scrap of plastic into place with a soldering gun. Then sand and touch up with primer and sand again. I use this method on plastic auto parts all the time. The stress is on careful.
Go check out Lowrider bike magazine, they have a tramedous amout of model stuff in there and may be able to tell you where to get those cool little rims and things they use while making them. They could tell you the right stuff to use, or at least what they use, anythings fair game in my book.
Model stores sell sheet styrene and you could do a chop just like the real thing. One of the neatest things you could do is get some 3/32 styrene rod and use a dremel tool to perform some plastic welding. Use model filler for the fill.
I'll make tip of all your ideas, i used the dremel tool to do the chop, and then sanded it to a smooth surface, i glued the front roof parts, then i put coulk on the back part(its a 49 ford sedan delivery), then i painted primer, glued the inside (for save keeping), the coulk broke off after it dried. I was very mad after all that hard work , i wish/hope the putty works!!!