I think it's an adorable idea, but I don't think limiting the weight and size of them makes sense. We're not going to race them, they just need to look cool, and those limitations will limit what people can do with them (particularly, noone will be able to put nacelles on theirs!).
I think the only limitation should be that you have to start with the basic body, and you have to keep, say, 70% of the body intact. Also, it still needs the wheels and should be able to roll, if only in theory. If you can't get the actual car kit (they're available at hobby shops and K-marts all over the U.S.), then CG is certainly allowed.
Those are my suggestions. :thumbsup: