Back in 1967 when I had my Replica in 500cc guise, I had some problems with whiskering plugs etc. I was using the timing method of lining up by eye, the crank centre, the centre of the crankpin screw and a point that was the middle of the hexagon head of the rear cylinder holding down bolt. The solution was to use a harder plug and to retard the ignition a little. The normal ignition timing often quoted for Scotts is 35 degrees before TDC at full advance, but my belief is that this is a bit too far advanced. I would prefer 32 / 33 degrees on a road engine.
If we open up the inlet and make it breath better, we set them to about 28 / 29 If we put on a high compession head, then this speeds up the burn and we set at about 26 / 27. On a competition engine with better inlet, a big carb and a mild extractor exhaust, I use 21 degrees.
Make a setting tool by fitting a bolt into the outer of a two piece spark plug body. use this to find TDC accurately. For a datum, the short edge of crankcase RH top near the carb base can be used as a sighting line.
Mark position when piston hits bolt ACW and mark this, then repeat CW, take mid point of both marks on periphery of flywheel, this then is TDC.
Make mark with chisel for future use.
A 9″ flywheel gives 2mm for every 1 degree on the periphery, so for 32 degrees, use a flexible rule and measure and mark a point 64mm round the periphery of the flywheel. Now line up this point with your zero datum feature and you will have timing you can reasonably trust.
For those who want real accuracy, we have the periphery of the flywheel engraved every degree, but this is not essential for a road engine.
Again I hope that I have not rambled on needlessly and send my kindest regards. I think we should take some pics and make an illustrated explanation on our website.