for gearing,
higher clutchbell (pinion gear) tooth count = higher speed/less torque
lower spur gear tooth count = higher speed/less torque
think stock, those hit around 40-45mph ish? limitations for speed are mainly shaving weight so you can gear it faster without overworking the engine, getting a more powerful engine, or getting a tuned pipe.
do not get things like the supercharger and the NOS/CO2 system, these will either wreck your engine, or at the very least will hurt performance.