Will changing my capacitors for ones with higher voltage but same microfarad allow me to use higher voltage batteries
The simple answer is no. The whole system is engineered to use a particular supply voltage, there's no telling how many different things could go wrong if you overvoltage it by such a great degree (not just by 10% or 20%, by 160%!).

The less simple answer is that it would be possible. With some serious reverse engineering, know-how, and cash for quality parts, you might be able to safely overvoltage a cheap r/c car. Although, you would probably be better off just buying a better r/c car.