Sure- - - -since the pressure inside the shaft is pretty low and the rocker arm would keep it from being blown out anyway. A piece of brass rod, a drill press, and a file could be used to make a plugwith a couple of thousandths of taper to stop up the extra hole. Even a hand drill motor clamped into a vise would be sufficient. Then a High Speed Press (AKA "ball peen hammer") would be the proper installation tool. That's about as low tech as it gets!

Yes- - - -that area of the shaft is HARD! Tapping the extra hole would be risky, (tap breakage) and I don't know of anybody who makes solid carbide taps.
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!