That hole has a 1/8" pipe thread. It would be easy to run a tap into the hole and clean up the threads. Just put a little grease on the tap flutes to trap any shavings you create while chasing the thread.

There are several designs of oil connectors between the rocker arm shafts, and they're on Ebay pretty regularly. Depending on the oil passages in the cylinder head, use a connector that can be supplied from an auxiliary supply tube, if possible.
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!