1/2" is too big- - - -3/8" pipe plugs work very well, and if you use pipe plugs with Allen wrench holes instead of square heads the fix looks a lot better. There's no need to drill- - -just chase the existing threads with a 3/8 NPT tap. Tinker with the threading depth a little until the plugs tighten well just as they get flush with the head casting
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!