Thanks Bryan. I actually got the lock unstuck. Gave it one last attempt before despairing, and it freed up. So many of our projects go that way.
I've put together a first-need parts list and will call for prices tomorrow. Mostly small bits for the moment; but that can add up.
I'm still trying to figure out what the 'mystery hole' is, and what goes there, before I order some bits. The hole is at the far end of where the starter motor lays, inboard of where the clutch cable enters the case. I can't find it in any of the drawings I've got. It has an upraised lip around it, as if some sort of cap fits there.