You should list what bike you have..........there are more than one type of bike and engine.
You should also give more detail in what you are looking for in an after market product.......Stock/big bore/ported and polished/ what?
I have a X1 with a Big Bore Motor pushing 55cc's. Stock sprocket, tornado carb, intake with velocity stack, and running over 100 octane gas (don't use octane booster btw). With my setup I can take cags at the tracks. With my 25 tooth sprocket I blow past them on the streets.