The repair is pretty strait forward. You need to cut, grind or pry the old core out. Then grind the underside of the hatch back to clean glass. Cut a new core. Lot’s of core options, marine plywood, divinycel, Coosa, and other composites depending on what’s locally available. Glue the core in with thickened resin, again, you have choices from polyester, vinylester to epoxy. All will work. Use weights to keep things flat while the core bond cures. Then glass the underside of the core. There are some subtleties, but that’s the basic process.