That would be amazing!!!I am thinking that rudder is just calling out for being made from epoxy. Make a form and do clear epoxy like they do those tables. So it looks like you dont have a rudder, just a termite eaten stump.
Is Thixo preferable to just mixing up a cup of two part West Systems epoxy?That's a lot to cover!
I wouldn't add any inspection ports until after the [gentle] air pressure test.
Gelcoat "spider-cracks" can be left for last. We do have gelcoat experts here. (I'm a painter).
Chips and cracks in the coaming (splashguard) can be filled with THIXO. Use tape to control the amount applied, and--after curing--sand down to fare the surface. (Usually no need to remove the coaming--which can introduce more work).