Your best shot is to pull one of the current hatches, measure the hole, and then find an aluminum hatch which will fit that or slightly larger. Then do the cut out of the new hole--if necessary remove any wet core. If no wet core (after the cut out--you will have some now)--then slight undercut, epoxy seal the edges, and then drill the holes for screws--over size drill, epoxy plug, with slight under cut--and then pilot holes in the plugs, and secure the al hatches with sealant. Should be end of problem. There are many reasons to seal these decks, including keeping the fuel tank dry, any water or macerator pump (depending on the year), keeping water out of the bilge, plus preventing any further core damage.
Looking at size and type--Hamilton has the "Aluminum Deck Hatches HM-H1624A, Made from anodized 1/4" thick marine grade aluminum with stainless steel locking mechanism. Anti-slip surface. Water tight, installs flush on the deck. Easy to open and lock. OD of deck flange is 19.5"x27.5"
This uses 4 bolts, so you have to use their "T" wrench, but if you are not opening it frequently this might be a great alternative, and is a bit cheaper in the about $375 price range. (thanks to Sunbeam for the Hamilton link)
Great project, which is fairly simple to do--just $$ and time!