For initial cleaning, try a very soft interior detailing brush to gently remove the grit and dust on the surface (running a vacuum near the brush helps). Once the major grit is off, try using Plexus as a cleaner. Spray it on a good micofiber towel and then wipe on and polish the lens cover.

Plexus and Aerospace 303 are good for interior and exterior plastics (including headlight covers). Both were originally developed for the aviation industry and work very well with almost any plastic found in autos today.