You can use theseMr. Clean Magic Eraser pads to get some not so deep scratches out of your plastics.

Very fine, slightly abrasive texture that works like extremely fine sandpaper, but not abrasive enough to leave scratches. You can then buff the area to get back the *slight shine* that the plastics originally have.
I realize a motocross bike is meant to have scratches, but I am not quite ready to allow them on my new 2012 KX450. In time, but not just yet .............LOL.