Adrian, Just a mention about the clutch...as long as the small "half circle retainers" are in place and supporting the thrust washer behind the clutch drum, the clutch inner will be hard against the thrust washer as the centre nut is tightened. The only lateral support for the chainwheel is in effect the running clearance between the thrust washer face, the back face of the inner drum and the faces of the chainwheel. Too tight.....it drags and seizes. Too loose...it wobbles. The centre rollers are too narrow to prevent all the wobble. To quote a saying "They all do that, Sir" In practice as long as the chain doesn't catch the inner chaincase, that's good enough for now.
If you had oil return, oil was being pumped into the engine. If the PRV was blowing off too early, low pressure to the crank would mean possible seizure here. As it has been rebored, a better chance it is a bore seizure, as long as the pump and PRV are working OK.
I could be wrong, but that is where I'll put my money.
Swarfy.