Lightly sand or glass bead them for some texture and then clear coat them. But you must use gun clear. Spray bomb clear won't bite.

This works great on modern headlights too, when they get yellow and/or cloudy.

The surface not only looks as good as new, but the clear will cover deeper scratches that wouldn't polish out and is much more resistant to scratches and the like because it is actually harder that the original plastic.