It's not uncommon for the internal passages to get clogged. Back when there were a lot of these engines on the road, we would remove the head bolt adjacent to that plug in the head, pour the hole full of oil, and use a snug fitting dowel rod and a big hammer to clear the passageway. It's possible to blast a high-pressure pulse of oil down the galleries by smacking the dowel a few times with a BFH. If that didn't clear the problem, we would install the external bypass line.
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!