Check out--
www.rotdoctor.com. Read every thread on transom rot. You will learn quite a bit. The transom on my '78 23' CC was cracked in the corners and let water in. About 8 inches of wood was rotted. I cut off the top layers of fiberglass in the corners, used drills and a crowbar to dig out the rotted wood, dryed it with a hair dryer, replaced the wood with west system epoxy and layers of a woven/mat combo. It came out stronger than new! Good luck.