So, now that things are looking like I can proceed with actually riding this thing somewhat... I am considering (once back Stateside) going to clip-ons and a slightly different tank, with a nose-bowl fairing like an old Norton or AJS style. Since I have the Matchless as a more upright scrambler-style ride, would be neat to have a something more road-oriented. Now that the bike's tripled the horsepower, more or less, of the original, the clipons don't see so out of place.
Spare stock tank would get knee and clipon indents, with an offset flip-open cap. Same olive color I think since it's steel. Not going to spring for a new tank for sure. Clip-ons, and the race-style fairing possibly with projector lamps built in.
Till then, I have lots of real work to do getting it in perfect tune as it breaks in some more. I think the ignition timing is in the right ballpark now, but I'm concerned about ping timing it because of the quality of the fuel. Still searching for a good fuel octane boost additive here. Hoping at least to find a source for pure toulene which my engine guy recommends the same as any commercial product.
I found "lead replacer" and bought it, but it turns out it's only intended to simulate the beneficial effects of lead on valves, not to boost octane.