You need to provide better information. What 49cc 4 stroke kit? What size wheels are you using? Do you mean the current total reduction is 9.8:1? What is the total weight (You and the bike)? Do you have steep hills or is it mostly flat?
Those 49cc 4-stroke kits are seriously lacking in power!!! The average speed of those kits is only about 25 MPH.
The stock size rear sprocket is usually the best compromise of power & speed.
Using a bigger sprocket will give you more hill climbing power but will reduce your top speed... using a smaller sprocket will do the opposite.