I found the oil-filler cap age-welded to the cases, and unwilling to come off. I soaked it with PB and gave it some time to loosen, but I still managed to break it. I’m not liking the idea of drilling it out… I had hoped not to split the cases. But if I must, I must.
Pulled the battery and air cleaner, and both sides of the housing that covers the oil filler cap, and vacuumed out more dead insects and mouse nest bits than I care to think about. Found one dead mouse, reduced to a dessicated hide by time. Broke off one screw removing the battery holder, and will almost certainly have to drill it out. The air cleaner is shot, torn up and then inhabited by mice. The metal end caps were badly rusted too; must have been holding water. The air cleaner tube (that’s what the parts diagram calls it) has some cracks but might actually be serviceable, at least for a little while.
So far, the stuck pistons and broken filler cap are my only show-stoppers. I have prepared a dolly to receive the engine, but I think I’ll leave pulling it out for another day. It’s been a long one already.