I got the back motor mount bolts out! All it took was admitting to myself that they weren’t coming out whole.
I had spent the last several days repeatedly soaking the bolts in PB. I also tried application of heat, with no luck (though it did a fine job of burning off the oil). I considered all sorts of unlikely clamp arrangements to force the bolts out backward, but to no avail.
Then it occurred to me… remove the heads from the bolts, then use the nuts on the other ends to pull the bolt through the case holes. I used a stack of washers, to which I added as needed, to ensure that the nut never quite made it down to the point where the threads ran out. In short, it worked.
I slipped a spare junk bolt into the left-side hole before removing the right-side bolt, to support the engine until I’m actually ready to remove it. I’m not sure my provisions for removing the engine are adequate, so I’m going to rethink them for a few days before proceeding.
Still have a broken bolt in a case hole to remove. It’s a small one; I may give in and just cut the permanent nut off of the frame and substitute a normal tap.
In other news, my cousin notified me that he found the title, and that the bike is actually a 1966 model year.