What outdrive goes in there? If itís an omc, Iíd haul the thing to the junkyard. When it breaks, and it will at some point. Your screwed. Cause they went outa business decades ago. The only parts your gonna find will be off another junked outdrive. If itís a merc, carry on. I would cut out the inside and leave a flange as was said. Get all the trashed plywood out and use a plywood form to hold the seacast in. Pull off the form and refiberglass the inside. If it is rotten. I would be worried the stringers are too.