We too have Lewmar ports
, and were having minor leakage in heavy rain/spray conditions. I was considering replacing those gaskets that you mention when I encountered a chap who worked on hatches professionally, and he was kind enough to give free advice. In short, he strongly advised to NOT try and replace them! He said that it was nearly impossible to get the new ones fitted, even with his considerable experience. The gasket
has a sort of diamond-shaped tab which fits very tightly into the groove in the frame, and he said that it was extremely hard to get it threaded all the way around the frame, especially if there was any corrosion
in the area.
The good news in our case was that he also said that it was seldom degradation of the gasket
that caused leakage. He advised carefully cleaning
the sealing area of the rubber. If that didn't cure the leakage, he said that the usual cause was the small rubber seal that fits between the two halves of the alloy frame. He said to take a small sharp blade (like a scalpel) and remove that seal completely, working from the outside. Then after cleaning
from the ends of the frames, fill the gap with Sikaflex 291 or equivalent.
We've done this on several ports
now with complete success. For us, this chap was a godsend. YMMV!
Good luck with it
Jim and Ann s/v Insatiable II lying Church Point, NSW, Oz