:welcome: to iBoats . . .
While many part for boats are generic and widely available, certain parts, like a cabin hatch, are probably going to be specific to the manufacturer. Most boat companies don't support their past product lines for that long (31 years)
If the hatch was plexiglass (lexan), then you can probably have one fabricated at a local shop.
Great Lakes Skipper is probably the best place to start looking, but for a 31 year old boat there may not be anything . . .
eBay would be another place to try.