I am a vendor and also have a few Maxes of my own but here is from a personal view point: Stock seat sucks, factory exhaust does well but any aftermarket header will improve the performance (slip-ons do very little but weight less), there are a few diff jet kits and don't bother with stage 1 (dynojet has stge 1 and 7 - none inbetween), handling can be improved and a few options for that.
Seats options include:
Mine (Morley's Muscle)
UFO
Sportmax
Seargants
Corbin
Jet Kit Options:
Dynojet stage 1 and 7
Factory Pro
Mine (Morley's Muscle)
V-Boost options:
Sasy (adjustable)
T-Boost (3k on)
Dale Walker Holeshot Autoboost
a few others that are adjustable
stock (which is fine 99% of the time)
Handling upgrades:
Fork Braces - UFO, Superbrace, MEK, many others
Solid Mounts - OEM Royal Star Venture (99 model is OK to use when ordering)
Frame Braces - Sportmax, Over Racing, MEK, many others
Radial Wheels and Tires (many combinations possible but require rear rim at a minimum since none for factory 15" wheel).
Chrome!!!! (I sell it on exchange - LoL)
Of course that is the tip of the iceburg. You can do anything from chain drive conversions to big bore monster power engines.
Sean Morley