I'm reworking mine right now:
First pic shows the aft edge. The plastic trim piece was broken - I used a piece of trim I demo'd out of a job (the stuff that's used to trim the edges of the FRP wainscot in commercial restrooms). You can see the profile in my hand at the right below the hatch. I've seen this at Home Depot. Had to trim the back (long) side a little to fit up behind inside the hatch.
Second pic just shows the front edge - I had to make a new wood piece for the top. Also shows the hasp (dangling on a stick) that locks the hatch shut to the cabin door - found this at West Marine for about ten bucks (identical to the original, but stainless instead of pot metal).
Third pic shows dimension from back of the hatch to the ridge that catches the upper hatch. That ridge looks like it is just about exactly 1/4" high.
Forth pic shows overall dimensions of the complete hatch (24").
I agree, it looks like the hatch is broken off, but there's enough beef (plywood core) in the hatch to screw some plywood on under what you have left and add plywood (or probably better, Maco Joe's favorite stuff - StarBoard) to complete the hatch back to original size. Then you could add a piece to replace the ridge.
Mine's an '85, but looks like yours is the same. I didn't notice where you're located, but if you are likely to take a wave over the bow, you'll want to make the hatches seal pretty well. I took one and the cabin got plenty wet (so did my sandwich, sitting on the dash). I don't have a good seal system figured out - so can't help you with that.
I'm trying to figure out how to make my hatches removable - so they'll come off and go back on easily. Here on the Columbia River, you need to anchor in fast water to fish and the hatches are in the way of my anchor system - I'll be a popular guy out here if I can come up with a good solution.
Post more pictures if you have more specific questions - good luck.
