Where can I find replacement parts and upgrades
Not knowing what you have now or what your looking for, make that a very vague question.
Builders use sometimes a variety parts from other vehicles when building these cars
Replacement parts for what?. Suspension, brakes, body, engine. then the year of the chassis makes a difference as to what will fit and if you have a VW engine that to changes through the years. So not knowing what your looking for or what you have now makes it next to impossible to answer the question.
Upgrades to what
Engine, Suspension, brakes,??
Again not knowing what you have already and what year you have makes this impossible to answer. Engines changed, suspension changed, etc.