For the 1972 model the hatch was an option and was fiberglass with a fiberglass frame that was screwed to the top of the cabin over a cut-out hole. No real problem with adding an "after-market" generic hatch to a curved section of cabin top, except you will need to construct a frame to adapt the flat hatch frame to the curve. Most, if not all aftermarket hatches are pretty much flat and even the slightest bend to their frames will cause a poor fit and leaking.
You could give Rudy Nickerson at D&R Marine a call (508) 644-3001 he was the Parts Dept Manager at O'DAY (1973-89) and now runs a little shop supplying parts to O'DAY owners. However, he will not still have the molds for that old forward hatch for the 22 (or if he does.. I'd be VERY surprised!) but he may have some good advice for installing a newer hatch and maybe a better price than other suppliers.
The later (1977-83) models of the 22 had a nice flush-fitting forward hatch, but not really practical to try to retrofit that to your 1972 without major surgery to the cabin top.
You might get lucky and find soemone with a 1972-76 22 with the optional hatch and willing to make a mold for you..... labor intensive.... but should be easier to fit?
If it were my boat, I'd just find a suitable new (or used from a marine consignment/surplus shop) modern hatch and build a level base for it on hte cabin top. Not a sleek as the originals... but easier!
I've included pics of the original optional hatch (outside-closed, inside, and outside-open) plus a 1977-83 example.
