Just did a similar repair on a MacGregor
21. Cut out the rotted area, cut plywood
to fit the hole, (hold it up from the bottom first with duct tape while you glass over the top), epoxy
in several layers of fiberglass
top and bottom, spanning well beyond the join line. . . . Sand to make it look kinda smooth and not terribly out of whack. Redrill and bolt on your mast step. The compression
post inside will probably need to be shortened by a hair to accommodate the new layers of glass.
Not too hard, just messy and time-consuming. You can make each successive layer of glass an inch wider than the previous, so it will have a built-in taper instead of a big bump/edge.