Deck hatches that develop soft spots or weakened cores are not uncommon. I can't speak for your boat, but in 99% of the cases it's not a design flaw. Most times it's due to a combination of owner neglect (not resealing hinges or deck plates on a regular basis) or simply due to it being 35 years old. Moisture has a w ay of finding it's way into tight spots!
Starboard is not structural. It would need to be braced and reinforced quite a bit.
Rebuilding a deck hatch is simple and retains the normal "look" of the floor. Even more so since it can be removed from the boat to do it. It does take some know-how, but it's not "hard" to do. You could even take the piece to a local glass guy and they can rebuild it for you. Composite cores are an option.