Many thanks for the list of items to check – really useful stuff.
For the record the compression ratio I quoted was calculated from the swept volume plus measured volume at tdc (approx 30cc per cylinder) divided by the measured volume at tdc. I now realise that this is quite misleading as the port timings (and the other factors you refer to) prevent a complete fill of the cylinders. Even so, by applying the same calculation to my other Scott engine this new one still has a relatively higher compression ratio.
I’m fairly confident that the flywheel and flywheel sprocket are sound. I will double check the clutch as i’ve had it apart several times trying to adjust for an even lift. One simple thing I will then try is to swap the NGK B6ES plugs for some cooler ones as the noise reduces when I richen the mixture. I’ll let you know how I get on but with these darker evenings approaching it’s getting more difficult to take the machine up and down the road during the week (no lights on the bike !!).