Hi!
I'm not familiar with the setup on the 8.5, is it custom fitted to the opening? Is the opening in the deck flat and does it have a wide enough section that you could bolt a hatch frame onto it?
The 8.2 has a flat opening with a hatch bolted onto it. I'm intimately familiar with it because I just replaced the old hatch - it was looking ratty and leaked like crazy. If the same option is available (requires about 3/4" wide flat lip around the opening to fasten the hatch onto), you can get custom sized hatches or if you are lucky (as I was) one that happens to be the right size. marinershardware.com
If - as I'm guessing - that's not an available option for you, and the existing fiberglass is unique to the 8.5, I'd say the options are to modify the deck opening (yech!) or to get a new hatch fabricated in fiberglass. Using the old hatch as a template for a mold, any reasonably reputable glass shop should be able to make something up.
Good luck!
- Graham