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There is a tool,I don't remember what it's called, but it is a hollow drill bit and you drill with the screw threads inside the bit and when you can grab thebroken screw shaft just unscrew it. You then fill the holes with epoxy and I wouldn't use the Home Depot type that comes in a double syringe but a good marine brand. there may be some rotten wood in there as it sounds like the stainless screws suffered crevice corosion from not having been bedded in a marine bedding compound like 3M 4200. You may want to soak the wood with Gitrot (or Get Rot). If you use the right sized tool to drill out the old screw shaft the epoxy filled holes will be hidden under the fitting. Now drill for new screws. Use a marine bedding for the fittings and on the new screw threads. Sometimes you have to use a lot of epoxy to fill a void. If there is a large void I have used the foam type of insulation in a spray can with a tube you can stick in the hole but leave an inch depth in the hull for epoxy. This may get me in trouble with some of the guys but I've done it and it seems to work ok. You can chop up some fiberglass strands or use cabosil to thicken the epoxy a bit but as long as the holes aren't much bigger than 1/2 inch I wouldn't worry about it.