Wetsand those suckers. Since you've cleaned them up with PlastX already, and removed the oxidation and grime, now you need to smooth the plastic down. Go to the store and get some 800, 1000, 1500, and 2000 grit wet sandpaper. Throw it in a bucket of water overnight. Then wetsand them starting from coarse to fine. After you wetsand them and they're good and smooth, they are going to look a bit hazy. Then take a powerball for paint (trust me, it works), with a bit of polish, smear the polish around the headlight before turning on the drill to reduce slinging, and polish the light out with the powerball. After you get that sucker crystal clear you then run into a dilemma...how to keep it from happening again. You could get the 3M film to cover the headlight. It works, it relatively cheap, and if it gets messed up, you just peel it off and replace it. That would be my choice. Others like to spray the lights with a few coats of clear for a bit more durability. My problem with that is the fact that in a year or so, you are just gonna wind up sanding and polishing again. The choice is yours, but if it were my car, I'd put the 3M protective film on the lights once you get them polished out.
And if you don't want to go out and buy everything separately like I have listed, you can also buy this:
Diamondite® Clear Plastic System Kit cleans, polishes, restores headlight lenses. Diamondite plastic polish scratch remover for headlights.
it's not 3M, but diamondite makes some great products, and that kit has everything you will need.