you should be able to see a manufacturer on the glass or the rim....if not you will need to measure the radius of the "corners" as well as what type of base your hatch has.
If you are not able to find these, I would call hunter for their help. On my boat, I was able to get the newer (flimsier, but fit well) version of the original hatch - which saved me tons of time in cutting glass and redoing the opening.
If it is just "leaking" you can get your original repaired - check the archives for the place in New England. Their prices seem to be about half the cost of a replacement and their work really good.
best of luck...