Jam, Was on line yesterday reading about window replacement with plexiglass. (Google search "lexan plexiglass on boats".) Appears to be a viable way of DIY repair. Remove old windows, take to a plastic's provider (like TapPlastics) and they cut you replacements. Refit into frames and seal. Appears the sealing is the issue. Nothing is perfect. Also heard that commercial boats use tempered glass. Issue is the weight. I did see an offer to polish on sight by a guy from New Zealand. Problem with polishing plastic is the change in clarity. The polishing will create waves in the plastic as you remove plastic to get to a "smooth" surface. I have attacked minor scratches with a combination of Meguiar's M17 Mirror Glaze Clear Plastic Cleaner and M10 Mirror Glaze Clear Plastic Polish. Cleaned up the plastic cover on my boat clock. A lot of work but resolved the small scratches from 30 years of abuse.
Hope it helps.