Thanks all for your comments. I have now sorted out the contactor arrangement for the ignition system. It consists of a fixed insulated sprung brass contact mounted on the engine body, and a rotating contact, in the form of a Tufnol disc on the crankshaft, locked in position with a brass screw, the head of which acts as the contact point and the tip of which provides electrical continuity with the engine body via the crankshaft.
At first, I threaded the hole in the Tufnol disc, but found that the thread stripped easily when tightening the screw sufficiently to lock it securely to the crank, so added a captive nut, as seen in the attached photo, which allows the screw to be tightened much more securely. By simply slacking off the screw, the disc can be rotated as necessary to adjust the timing.
I have also attached a photo showing the underside of the gas/air mixer valve where the air holes can be seen. Gas enters via the side pipe and through the valve seat, as with the air holes.
Just have to sort out lubrication for the big end, then should be ready for a trial run, bearing in mind your warning, Graham, to ensure that the engine is bolted down securely. I have tested the hit and miss mechanism by spinning the engine over with an electric drill and it appears to be working well. Nonetheless, the kill switch will be close at hand!!
