too small and short ports, too small carb, and restricted airbox.
Stuff you can buy: Jemco pipe, Boysen reeds, UNI air filter, PW50 gear, Mikuni VM18mm carburetor, Holeshot clutch spring kit, if you want you can even do a 60cc cylinder swap
Stuff thats almost free: Added Base Gaskets, if you run 2 or 3 or maby even 4 you will raise the exhaust duration which will in turn increase your rpms, drill holes in air box and upjet, port the cylinder, loose weight
You can get parts at http://peeweecycle.com/MiniracerHome.html
Most of the stuff for a PW50 will fit your engine