Parts are pretty available, mechanically speaking. Obviously some harder to find than others. Some parts from CRs and a few other machines are also interchangable.
Yes you can buy plastics, used and new, Maier. Tanks are a little harder.
Fork swaps and upgrades are pretty easy. Yes to the later Rs and Xs.
If you need stock parts, Manbearpig here on the forums is very helpful. Haven't seen any posts from him in awhile. Anyone know what he's up too?
81 250R Custom
83 250R (in progress)
98 Scrambler 400
03 Bombardier Quest 500
07 Polaris Predator (sold to klondike1020 )
68 VW Dune Buggy