Go find a West Marine Supply store and get a bottle of plastic polish. You use it to remove scratches and oxidation off of lexan and plexiglass windshields on boats.
It should be alot less than $30 YOu may have to use scrap cloth on a stick to polish the insides.
Be aware that heat after time will also cause plastic to be brittle so make sure you dont break the lens. After the lens is polished you can wax it with a quality carnuba and that will slow down the oxidation rate.