This is how I cheaply restored the plastics for this bike.

As you can see, the plastics were all chalky and white. I used a razorblade scraper to remove the top layer of old plastic until the color was full.

I then used some plastic polish and an air buffer to polish the plastic.

The tank was discolored from fuel and sun light. Same process, scrape and buff. I think I could have done a better job by wet sanding before buffing. But this was good enough for me.