There is a product called Novus which is essentially an ultra-fine rubbing compound made for taking care of plexiglass windshields on aircraft. It comes in three different grits, #1, #2, and #3. Long ago I bought #2 and #3 to take care of scratches on my PC800 windshield but didn't buy the #1 because normal plastic polish can do the finish work. If the scratch or scratches are deep, you'll have to start with 400 wet & dry sandpaper then progress to 600 wet & dry then use the Novus. You can buy it at Whitehorse Gear plus it's also available at other places.
Last month I used those two + normal polish to clean-up the dirty and grayed headlight lens on my lady friend's '01 Suburu. It did a great job and she was overjoyed that she could again see the headlight pattern on the road! Prior to that I couldn't tell if her headlights were on when riding in her car.