I have one of those in my garage right now. My friend bought it as a basket case for his son for Christmas. I have it stripped down to the frame. The frame, swing-arm, triples, pegs, bars, etc have all been sandblasted and painted. I went over the engine and had him order seals, gaskets, etc. You can get a complete plastic set for $139 on eBay that also includes a new seat and air filter for these PW80's. No pictures yet though. I will take some when I start the re-assemble.
You're not going to have much luck with that plastic. You can try scotchbrite pads, fine steel wool, or fine sandpaper. Some people suggest using a razor blade to lightly scrape the discolored layer off but I never liked that method.