I've done the rudder bracket repair on numerous Sunfish. Easy repair after you get access to the back of the bracket.
One of the two of the best ways to get there are using one of these:
WEST MARINE 8" Screw-in Deck Plate or the six inch one. The size of the hole does not correspond to the size of the deck plate, and not all deck plates use the same size holes for a given size. Something to be cognizant of prior to cutting the hole.
Buy or rent, borrow, or steal a hole saw to make the hole. Alternatively, a router can make a pretty good cut.
After you cut the hole a $20 bathroom fan and a length of exhaust hose can be used to move air way up into the unreachable caverns and voids within the hull.
Save the piece of deck that comes out you can use it to do the second method which is to cut the hole and fiberglass the cut out portion of deck back. We did not do that often as the boats were used for sailing lessons and far from pristine so not worth the time and effort generally. Occasionally the fiberglass guy we used would be there working on a boat in the marina and he would glass one back in while he was waiting between coats or for his project to cure. He could do the whole deck repair in about a half hour, and it would be difficult to even find where the repair had been made.
So, save the cut out section even if you do the deck plate you may want to glass it in later.
Another tip I've been given if you are still trying to get water out of the hull is to drape the boat in black plastic and set it out in the sun. Keep a gap between the plastic and the hull as the heat can cause the finish of the hull and fiberglass to separate and bubble up. This happened on a couple of boats we had "For Sale" signs taped to the hull. Cut holes around any access ports to the interior stick the vent hose with a bathroom fan as far as you can into the hull and circulate air.