[BAracer]

As per previous responses, what is your aim for the car?

Any car has a certain performance envelope based on its geometry and mechanicals. You could put a more faster motor in it but all you might end up doing is having to drive it at the same pace as before to avoid it tipping over on every corner. 

In my limited experience of rally cars, it you want a truly fast one, buy a SRC. But I also have some SCX and Ninco 4x4 rally cars in their original spec, save for some softer tyres and some ballast, and they can show an impressive turn of speed once you get the tuning right and bed in all the gears.