Wet sand with 600 grit, then 1000 grit, then plastic polish and a lot of rubbing will bring back more shine than thought possible. It works! Have done this many times before on these headlight covers and old faded tailights.
For almost that cost you can just obtain new headlight covers from Altec Products. They will sell direct and beat MSA's cost. Ask for Greg. Here is the link...
http://altecproducts.com/products_zzxlenses.htm