If you are looking to upgrade a engine and have fun doing it get the 2stroke kit. Dont buy one from bikeberry they tend to have bent connecting rods.
There are many things u can and need to do to these engines to make/let them perform at optimal performance.
You can start by breaking in the engine dont be hard on it for a couple tanks of gas. Unfortunetly the cylinderwalls score easy and get damaged easy, not a big deal because a new cylinder is 24 bux piston and rings are 14 bux.
After you break in your motor and are getting a feel for what its like and all that jaz then you may conciter looking for performance. If you are handy you may want to look into porting the cylinder as they are pretty rough. There is a big veriaty of performance cylinder heads, the stock head does not keep the engine cool. Look into a "fred head"
You can get a different carburettor. Dont waste your time with the CNS carb its junk look into dellorto clones.
You can get a expansion chamber exhaust from sickbikeparts.com or many other sites thats a must have.
Different size sprockets for torque or speed
You can upgrade your cdi to HD lightning or a jaguar
You can buy a shift kit for mtn bikes at sickbikeparts and shift like a motorcycle.
There are endless things to do to improve everything. A lot of machineist make custom parts such as sprocket adapters, billet heads, chain tensioners, clutch covers an more.
Do your research on google, any question you have there is a pretty good chance its been answered in a previouse thread.
If your looking to build a monster you will end up shelling out hundreds like my self and a lot of other members on this forum lol
If you look on the left and right of this forum page there are advertisements for 90% of the motorized bicycle stores.