So I soaked the engine for a bit longer. Managed to loosen off the valve caps so I could remove the valves and springs, which need to be out of the way to get to two of the barrel base nuts.
Now this engine has sat a long while and I was a little worried that the barrel and piston would be shagged, cos finding replacements would be impossible.
The barrel came off easily, which was a relief:-
Feeling inside the bore and its nice and smooth - no scoring or rust damage. Thank fuck for that.
Removing the barrel revealed a nice cast iron piston in great nick:-
Gotta be really careful with this one cos they are pretty fragile.
The big end moves OK with no signs of wear but the crank won't turn. I reckon the main bearing is rusted to hell. Still can't get the bottom studs out so will need to point the drill again.