G'day Taz, first thing, derestrict the variator and cdi if they have not been done yet, though at that age I would reckon it has been done. To derestrict the cdi on the ZX, bridge the light green and dark green wires, this will eliminate the rev limiter. If the drive boss has a shoulder on it, then that will need to be machined off to allow the variator to close fully, giving you your top speed. With regard to upgrades there are a number of options. First the variator rollers, you could try lighter for better pick up, clutch fly springs are another option, I am running 1500 fly springs (higher revs before take off). Cylinder kits are abound, I bought one from an ebay seller, cheap price, and poor quality to go with it, see my 82cc upgrade in this section. Most people suggest Malossi or Polini, I think Polini is the one to look for. A high flow air filter with a larger main jet is also a step in the right direction, as for exhausts, Technigas is supposed to be up with the best though I can't speak from experience on that one.
If you have bought the scoot for something to play around with, you are in for a lot of fun, it just seems to be endless what you can do with them. My mods on the cheap have been a bit of a headache, but that is what learning is all about.
Welcome to the forum, lots of people here willing to help.