C&C Yachts - C&C Photo Album & Resource Center
is a place to start... Down the left side of the page is a menu including DIY (incl hatch replacement) and members forum.
Keep in mind you can get hatches in a surface mount or a flange lip configuration (the latter has a lip that slides into the deck opening) Perhaps you should be looking for a surface mount that is more forgiving for slight offset in hole size. The lipped hatches provide a more finished look inside with less work.
C&C may have produced their own hatches but my recollection is that they used Bomar, at least for a time.