One thing to be aware of in trying to remove one larger scratch, especially a deep one: As you tediously sand away to remove the scratch, you are making a small depression in the surface of the plexiglas. That becomes in effect, a small "lens", and can result in distortions, or "waviness" as you look through it.
The US Navy tried repairing F-4 Canopies that had been scratched, and made them MUCH worse, because of the waviness. More than a few had to be replaced after the "repair".
Use caution, and try it on a piece of scrap plexi first.