I have an 89 Sowester 2520. I just had my tank replaced last month. It had a coating on it, but had degraded over the past 30 years and had pitting in the sides that were very close to causing Leaks. The issue is the open cell foam that Parker put in the hull between the stringers and to set...
I still have these windows available. If they are not what you need. I believe that the windows in your model are made by Diamond Sea Glaze. here is a link to their website. They will make you anything you want.
diamond-sea-glaze.