I would focus most of my attention at the frame and suspension before more power... Most people are so focused on going faster but they don't realize the bike itself restricts them from going fast.. Same thing as on a big bike.. Suspension work is more important then power most of the time.
Buy a factory service manual for like $20-40 and do it yourself. These little engines are so easy to work on.. Very few tools are needed and even if you need to purchase a few like the clutch nut socket, torque wrench, ect ect you will still be ahead $ wise and you'll have some new tools!!