wooden hatches on ericson
Usually the forward hatches are the easy ones to replace. You willneed to very carefully measure the opening and the "flat". If you look at how a hatch is made, there is the a piece that gets bolted down to the deck. That piece has the actual opening which will need to be pretty close in size to the opening through your deck. This same piece has a flange that sits against the deck. This piece has an outside dimension and a bolt hole center dimension. To pick a hatch you need to make sure that the interior dimension of you deck opening is not too big or too small, that the bolts (or screws- bolts are preferable) end up passing through a solid amount of deck, and that there is a flange that is large enough for the hatch to sit on. This may require some modifications to your boat. The most probably problem will be too small a flat. Boats of that era often had a molded spigot (a raised lip that turned up inside the hatch cover). and these were typically 3/4" wide or so. Hatch bases are closer to 1 1/2" wide so you may need to either cut off the spigot and reglass where it was located, or widen the spigot which also involves some careful glass work.
There are some companies that make sliding companion way hatches, but a fit is unlikely. one thing you could do is to use your old hatch a plug and make a fiberglass mold and then mold yourself a new f.g hatch. That is not that hard to do although it takes some time to make.
I have used Bomar and Lemar hatches in the past. Bomar Hatches do not appear to have a website but Pyacht (http://www.pyacht.com/) apparently carries the entire Bomar line. Lewmar has a site, (http://www.lewmar.com/) Lewmar''s site includes a catalogue. The Sailnet Online Store may also carry both brands.In all cases you will need to send your outside, and inside dimensions to them and ask if they have a product that size since these sites do not include detailed info on product sizes.
I would take some careful measurements of the existing conditions on your boat and then look at these sources. Good luck