Those holes look like 1/2" NPT. That's an easy drill & tap job, even with the head on the engine. Just put a little heavy grease on the tap flutes to catch most of the shavings.
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!