According to Des Perado :
How about a fiberglass patch? You can buy kits at you auto supply
store quite inexpensively and it might be a lot easier than cutting
metal, drilling and riveting, etc. and it won't rust.

My first thought was fiberglass as well.

However, the other comments about the rest of the deck rusting suggest that
before considering _any_ sort of patch, you need to evaluate the whole
deck first. If the deck is nearing collapse, a patch could make the result
more dangerous. A large hole should be considered unrepairable - especially
one that "moves". Patches generally won't add much in the way of structural
strength.

Standard automotive polyester resins are fairly brittle. You'd
have to build up several layers of glass. Also, they're not going to
stand up to abrasion very well - which is going to be an issue especially
if the patch is in line with the blades.

Epoxy-glass would be better.

In either case, you might have quite a problem cleaning the deck well
enough to get good adhesion.

If it was deemed repairable, I'd consider riveting aluminum sheet on the
inside, and then buttressing the patch from behind (outside of the deck)
with a couple layers of fiberglass to rigidify, seal the result, and
deaden vibration.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.