I'm posting this in the hopes that it might be useful for someone else. This cost me a few hours of my life.
Some previous owner of the donor motor I'm swapping into my bike used /way/ too much Orange RTV gasket maker to reseal the oil pan. It was pointed out to me that if so much had been pressed to the outside a similar amount would be pressed out on the inside and over time would likely break loose and contaminate the engine.
I undid all the mounting bolts and whacked on it with a rubber mallet with gradually increasing force.
I tried a razor blade.
I even tried attaching a ratcheting strap to the oil over opening and cranked on it as hard as I dared and then whacked on it with the mallet.
Based on suggestions from the K100 group on Facebook, I tried the thinnest wire I could find, a light E string from my guitar. It was too thick.
Based on another suggestion, I tried dental floss, of all things.
That allowed me to make a small bit of progress but it would get stuck and build up.
I tried strands of high strength nylon rope but that did not work as well as the dental floss.
I wanted to see what thickness of wire might fit into the very small gap so I pulled out a 0.005 feeler gauge. It then dawned on me that maybe I could use the feeler gauge as a cutter carefully.
It worked surprisingly well. I did put some scratches into the mating surface but my suspicion is that I did not do enough damage to cause a leak.