nice choice, i have the same bike.
things you would want to do to it first is
1. fix the rearhub sprocket bolt setup.. find a bolt long enough to go though the hub and put a nut on the end then loctite it so it wont come loose.
for performance you can't really do anything with that motor without killing its gearbox. but an oko 26mm carby tuned right would make it alot more responsive and snappy then the standard mikini 25mm, dont need to swap exhaust because its a good pipe and good size. (best is a 28mm pipe and 48mm muffler tip from memory, which is what it has)
also do valve clearances if you haven't and clean carby after every ride.