David,
I'm going to do as you suggest, a leak down test. I also read in Richardson's last night about 1) bypassing the rocker gear temporarily to determine if excess oil is finding it's was into the combustion chamber via the valve guides. If the smoking stops then it's the guides. 2) Temporarily soldering up the oil jet that feeds the cams and back of the cylinders, if that stops the smoking then the engine requires restricter discs, if it doesn't have them already. According to Richardson the jet can be as low as 140, but he recommends not to deviate from the standard 170 and control the flow via the discs. KTB says the hole size in the discs is 0.025 - 0.30. I know some on this forum have said they don't even drill the oil way in the liner, so gradually reducing the hole doesn't worry me massively.
So:
1) Check breather assembly is still an assembly.
2) Leak down test.
3) Bypass rockers temporarily.
4) Block oil jet temporarily.
5) Check rings.
Watch this space for results.
Steven