I had previously made arrangements with a well-recommended local indy to replace my leaky oil pan, so was already prepared to open up the engine. The core that he was getting the oil pan from has a good bottom end.

The plan now is to remove the engine and have a good look inside, then determine the course of action. We can use the crank from the donor engine if need be.

The engine still starts right up and feels normal, no bad sounds or irregularity other than the oil pressure. It has seen minimal use since the problem appeared, and by driving VERY gently I was able to keep the oil pressure at a safe level, so I am hoping that the damage is limited. It is parked now and will stay that way until repaired.