How do you make a 50cc dirt bike whit a weedeater motor in it go faster?

i am using a 29cc weed eater engine in a small peddal bike as a proto type for my 100cc chain saw engine bike. use a chain saw motor it is better but keep most of the plastic b/c it flowes the air over the cylender but if you cant get a chain saw then make the bigger sprocket smaller but you loose power!!! MUST HAVE!!!!!!!!!!!!!!!!