So, after soaking in Plus Gas the stud came out. Result - one loose engine.
Still need to drill out two more studs at the bottom of the crankcase as they're seized. Not sure how easy getting the barrel off will be. Cast iron piston in a cast iron barrel...hmmmm. Its at the top of its stroke and both valves are closed so hopefully the corrosion is minimal. We'll soon find out. Plenty of Plus Gas in there at the moment.
More to follow.