You don't give any info on what cylinder you currently have ect, nor your definition of "max". For example, you could weld and machine the crank case halves, stick on a 75cc cylinder, 21mm carburetor, simonini exhaust and a hpi ignition. That would be pretty close to "to the max" Then you could balance and modify the clutches, port the cylinder and adjust the gearing. That should run you around $600 or so in parts, but with the right final tuning could be one of the fastest tomos in the states.
Jets control how much fuel go into your engine, if make the engine flow better or require more fuel you'll have to install a larger # jet. Exhausts, air filters, carburetors, bore kits, all of these will require a larger jet. The correct size jet is important also, too small and you'll run lean (lose power, run hot, and possible engine damage) too big and the engine will run rich (lose power, foul plugs, excessive carbon).