Tom.. assuming the old BH is intact enough you could use it as the pattern for the new one? Or, make a template from it out of 1/4" plywood and use the template with a pattern bit in a router to make a new one? On removing the old one maybe it's a good idea to make sure the BH is not under stress, like the mast and
rigging being under tension. Possibly you would need to support the cabin top while the BH is out. I've a 30 Oday don't even want to think what it would entail in my boat!