Your best bet as Steve-0 said, find replacements. If that proves hard to find I would fill the holes with J-B weld, re drill and tap after very carefully cleaning the stripped hole so no oil/contaminants are on the metal. Another option is get Helicoils, cut them to length, insert with J-B Weld and clean up the threads after the epoxy sets up a bit so you have a clean thread. You woulld need to make sure the Helicoil is in far enough so the O ring that seals the plug has a place to seat against the bowl.
-P.