Bomar makes a 20" x 20" hatch which should be a direct drop in for the hatch you have. If you want you can also go up in size.
Be sure and remove any wet or delaminated core around the hach cutout. Fill this with thickened (Cabosil) Epoxy between the two deck skins of fiberglass. Then you drill new pilot holes for the screws and put some 4000, boat life life seal, or Dow 795 sealant around the hatch as you screw it down--us masking tape on the deck and edge of the hatch, so it does not have to be cleaned off after the sealant sets up. Take some photos.