I also have one of those cars, but I have the Red wide body
I was looking at doing the mods to it also.
If steering/suspension etc breaks, then you'll most likely have to buy from NitroRCX otherwise modify other manufacturers components.
As for motor and esc, just do yourself a big favour, go out and purchase an eZRun 9T Combo, for around $90.00 you can't go wrong.
The motor limit on the standard esc is 23T as far as I know. Don't waste your time or money trying to fit a different motor to the current esc, it's really not worth it.
If I were you, I'd just save some money and buy a new Yokomo chassis and add the extras that you want, to it.
The Exceeds are a good learning car, but are nothing compared to the Yokomos, Tamiyas or HPIs.
Hope there is some relevant info umonst that somewhere lol.