Well, what type of bike, which 66 or 69cc engine do you have, or is it the 80cc 4-stroke? Are you still doing the break-in stage with the right gas/oil ratio and rich fuel/air mixture? What type of terrain are you riding in?
Pictures of your build would help too.
There are numerous build threads and albums already posted that you can draw ideas from. (Mine is not too shabby as a collection of mostly other (better) mechanics' ideas applied to the cheapest generic no-name engine)
But I would start by ensuring that the bike is sturdy and safe for the excessive speeds, and that you have adequate safety equipment and bike riding skills, before doing anything else.
Then the engine should be very securely mounted, and the drivetrain should be reliable, effective and efficient.
I would start on the bike safety, the reliability mods to the engine and whole bike, then look at the other improvements you can do.
When you have the break-in finished, then you can get the fuel/air mixture dialled in nicely and that will help your speed too.
