Get the next size larger allen wrench. Cut off a straight length of wrench. Grind one end slightly tapered so that it begins to go into the hole in the stripped bolt. Use vise grips to hold it and tap it into the bolt head with a hammer. As you are tapping, turn the wrench with the vise grips.
You should be able to get replacement screws at any good hardware store.
Replace the gasket and do not overtighten the screws or you will cause leaks.