I think a plain old 1/2" pipe plug will keep it from leaking. Just put a little pipe dope or liquid Permatex on the threads as you install it. Some engines use a 3/8" pipe thread instead, so you've got a 50-50 chance of guessing right the first time.
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!