It only takes a few minutes to run a pipe tap into those holes and do a permanent fix that doesn't leak. I don't remember if the proper size is 1/4 NPT or 3/8 NPT at the moment. No drilling is required- - - -just chase the existing threads with a tapered tap. This can even be done with the head on the engine- - - -any chips that fall into the exhaust port just get blown away from the port into the exhaust pipe. I use Allen socket pipe plugs to fill the holes.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!