I just printed the drawings and had a look... They are helpful, but don't tell the whole story;
- the picture you attached at the top, that must be where the support rod attached to the bottom, correct?
- what do the chainplates look like up on deck from above, and what do they look like inside but above where the rod attaches? I'm keeping my fingers crossed that they make this out of stainless.
The main Chainplate bottom supports (the part on page 3) is what was made of mild steel angle and then galvanized, which I believe is the part that has rusted out. It would be easy to make a new one in terms of fabrication, the hard part is installing it.
Its one solid piece that is under the grid structure and the tabs protrude through slots where the bottom of the tie rod connects.
- To get it out, did they glass it in (probably), or just use adhesive or what... big question.
- To get it back in, is there even access to that area and how much stuff is in the way?
Making the new part isn't complicated at all, it's getting it installed that could be a real bear. This would for sure be a rig down job, plus a good amount of the interior might need to be disassembled to have access.
The new supports could be made of stainless steel, but the material is 1/2" thick. It will be expensive for the material and a royal pain to cut. My plasma cutter will do mild steel that thick, but not stainless. I have to buy the bigger unit to do that. That's probably why they used mild steel back then, they didn't have plasma cutters.
Another option would be to make composite replacements with G10 board and glass it in from below. Believe it or not I think it would actually cheaper, less labor in fabrication. Big question still is, how to get the whole thing in place under the grid structure and glass it in.
Can you take some more pictures? Take a shot of the chainplates from the deck, and then also looking up to the deck connection from the inside. Then, can you get below the grid and see underneath where those tabs come up through the deck? If you need to, you might want to get a camera on a snake that attaches to iPhone or android phone. They are pretty cheap on Amazon. Getting one myself soon. I wish I lived nearby, I'd love to crawl around in there myself to help you figure this out, I love a good challenge!