Something that I have personally found works good for a quick cleaning is the Windex, I think multi-surface, that has a lil vinegar in it. I used it cuz I was in a bind one day and needed something simple and quick, and it worked great. I only spray it on a towel though and wipe the stuff down, I don't spray it all over the car because it can't be good for it as far as getting down in the cracks and grooves and all that. But if you do need to wipe down the chassis or aluminum parts and stuff like that to get the grease and fuel and mud off, it has worked great for me. Just be careful, I always try it on the bottom or something thats not seen to make sure it will not hurt the surface.
If you have a LHS nearby, you may want to pick up some nitro-cleaner. Its especially designed for all of the surfaces on an RC and it breaks down the old gunk left by fuel really good.
I know you weren't happy with the performance and quality of the first car that you bought, which is why you came on here. But since you already have it, you should put a cheap motor in the car and get it running again and then you and your son can run them together and both have a lot more fun for a lil while.