Well, this is all taking a lot longer and costing much more than I originally imagined. A quick update:
I never managed to get a non-buckled 06.1916 lower yoke, but found someone locally selling an unused MK3 frame that he'd inherited from a Norton importer. Certainly looks unused other that a few scratches from being dragged around concrete floors for some years. There is no VIN plate nor even drillings to fix one so I have no idea of the real history. It came with an upper ANG yoke as he'd lost the lower one, so I ended up having to buy a new lower yoke after all. Just need new stanchions now, but a least it all lines up properly.
I got a couple of new cogs to replace broken teeth in the gearbox and put it back together only to find the layshaft was quite bent and the main a little. Ordered new from RGM, but ordered the wrong mainshaft (a Manx one, so if anyone wants it let me know).
The machine shop finally put the sleeves in the barrels but were worried about cracks in the lower skirt. I have someone who will sort it somehow.
I should have them back next week, after 4 months...
In the meantime I've been rebuilding the wheels following some of the great posts on this forum. I have some questions though:
Can I make a rim band using an old inner tube or is that too thick?
Some of the spokes stick out by a couple of mm - do I just grind them down with a Dremel?
Should I treat the rust (I simply wire-brushed it so far)?
Should I worry about the grit I hear inside the rim when turning it?
Looking forward to starting on the engine rebuild soon...