well the fastest is a variated bike but those are not the cheapest
the one with the most and cheapest performance options is a puch
since puch kits will work on a tomos those have a lot of kits also.
and if u happen to have a welder any performance exhaust will fit that puts all bikes = there.
carbs are a pain to mount on most setups not all but a lot of them. it is usually the hardest with tighter engines (most Japanese) or moving engines (moby and pug)