Carb - The bigger the carb (meaning bore and jets sizes) the more fuel you can introduce to the cylinder.

Reeds - Stiffer reeds close tighter and faster sealing your precious air/fuel mixture into your case. This aids in the pre compression stage of your mixture. Stiffer reeds also don't flutter at higher rpm, which creates turbulence before the charge reaches the combustion chamber.

Ported head - (this also includes the porting of the case at the transfer port bottom) this allows more fuel/air to enter the combustion chamber via lesser resistance and larger cross sectional area. (Up to a limit, this gets WAY more complicated) This includes matching the ports between the case and the head and smoothing any obstacles to the mixtures flow.

Indexed/matched piston - Indexing the piston (the way I do it) effectively increases the transfer port size at the top. It also advances the intake timing, allowing more fresh charge into the combustion chamber. An angled "index" will guide the charge up and to the rear of the chamber which will swirl and create a vacuum at the front of the chamber; this helps scavenging and creates more space for the return pulse charge. This makes your power band peak stronger and allows your expansion chamber to be more effective.
Matching the piston is cleaning up the underside to maximize flow and to enlarge the transfer port at the bottom, this is mostly effective on the cag piston port style motors.

Tuned pipe - A properly tuned pipe will create a return pulse at precisely the right time to pack the left over fresh charge back into the combustion chamber. At your power band peak this essentially acts like a turbo.

Performance clutch - I'm mainly talking about the clutch engagement time here. If a clutch is set to engage at later rpm, you can build more horsepower before it engages. This will give you substantially more acceleration. The motor will also rev faster at the lower rpm due to the lack of a load on the motor. This also acts as if you have a lighter piston/crank, until engagement that is.

Sprockets - Bigger rear sprocket or smaller front pinion = more acceleration, Smaller rear sprocket or bigger front pinion = less acceleration, more top-end
Pinion to Sprocket ration ~= 1:10, that means 1 pinion tooth ~= to 10 Sprocket teeth. If you're looking for a good all around gear ratio to ride with everyday, and you want some good clutch pad life, stick with your first thought...7/72 or even 7/74. You'll have much better acceleration, but not quite the top speed as a 7/66 or a 7/68 would have. Also, try a 6-tooth pinion. It’s easier for your engine/clutch (as far as these centrifugal clutches go) to push a 6t pinion than a 7. Try a 6/68 if you want.

Article source www.pocket-bike-racing.com.au