Try restoring the plexiglass with various plastic polishes, or give it a flash-refinishing with a blowtorch. I tried it on a pair of seriously messed up plastic sunglasses and it helped a lot. I just fired up the torch and passed over it quite fast about three times. Be extremely careful as it takes less than a second to heat the polymer up enough that it bubbles.
If the crazing is down into the structure of the polymer you are probably screwed, and replacing them is the only option.
Is it too difficult to manufacture your own replacement lenses?