How to remove scratches from LEGO windshield parts?
Haven't tried this with Lego, but here are the first two things that come to mind.

Novus plastic cleaners (they make 3 solutions for various scratch depths, with Lego I would probably buy the one for the finest scratches) work well on my pinball machine plastics.

About the transparency, I would try the standard headlight cleaning solutions for a car.