To increase the power of your motorcycle's engine generally requires some part of mechanical knowledge or deep pockets filled with money, on the other-side is relative easy to fake a engine power increase. The easiest, and probably the cheapest, and most effective way, is to change the final reduction ratio of your final drive. This can be done by changing your gear to race gear or simply by increasing the size of the rear sprocket, or reducing the size of the front sprocket. With changing your final drive reduction ratio you can give the impression that you motorcycle has more horsepower.
The downside of changing the final drive reduction ratio is that your motorcycle will not hit that previous top speed, and depending on how much you changed to the final drive reduction ratio your motorcycle can be much slower, but it will get there very damn fast.
The other option is to lose weight. This is trickier and often more expensive. The cheapest way is to lose a few kilo yourself, but that is most of the time easier said than done, the other way buying aftermarket motorcycle products. A motorcycle's performance relies on its weight, so the more kilo's you lose, the faster your motorcycle will perform. Remove you tool kit, take unnecessary stuff from your motorcycle. Buy an aftermarket exhaust system, this often safes you kilos in weight.. Chang your ABS plastic fairing for carbon fiber fairing... this all will help to reduce the weight of the motorcycle. Also do the fuel calculation, if you're racing on a circuit – calculate how much fuel you need to complete the track at fuel throttle, a few liters of fuel to much is a few kilo's to much.