Welcome to the forum... For your first question, you can do this if you know how an engine operates... basically, you have air intake, fuel delivery, combustion, exhaust, and the transfer of power to the wheels (transmission) ... If all are well tuned and efficient, you can achieve the optimum power without spending anything... Air filter is always clean and unrestricted, carburetor is tuned and clean, sparkplug is good, exhaust is unrestricted, rollers are good, and belt is good, cvt housing is clean, lubrication is fresh, etc.
For your next question, the first mod that may give a significant improvement are experimenting with different roller weights... lots of threads here in the forum... get the best weight suited for your needs...