Thank you for the advice, it all helps. It won't diminish my enthusiasm, but will make me cautious!
I am hoping people sell good stuff!
The original cylinder would need about a 1mm overbore to remove the gouges in the bore around the port where I suspect the ring broke. Someone in Italy took the lot off, cleaned it out, fresh rings on the same piston and off they went again. There was still bits of shards/ring in the bottom of the crankcase as that could not be cleaned properly. Further, the top cooling fin has been snapped off, about 40% of it. This must have been when the exhaust was removed and during the can opener episode to the cowl.
A friend has a hand honer so we can put cross hatching into the bore. The port edges need some tlc and I'll do a piston/head clearance check as a dry build before the full button-up of the top end.
The crank bearings that came out, 1964 vintage, are remarkably good and there is nothing in the rear hub bearing and no leaky seals anywhere. I'm leaving the chain side crank bearing in, changing the mag side and seal, and to be honest I'm leaving the gearbox as it is though the clearance under the top cover is 18 thou. I'll keep the bearings I have safe, hopefully not to be needed later.
I have a fresh new OEM clutch, 4 plate with new corks/steels and springs, new 80 link chain, new guides, circlips etc and seals.
Cleaned the mag casing today and all the threads for the cowl screws are intact!
All of the million case studs are out, gasket faces scraped clean and re-installed with a tiny drop of Loctite. Forgot to order new inlet and exhaust studs
I want to get all the new parts in and the covers on asap so the lot is tight and I can move to the carb, and then the big stuff, the frame and forks.