Bit of an update, engine started dropping onto 2 cylinders, then onto 1. Took the plugs out and it was fouling badly (oil). So I'm now running 50:1 with a set of new plugs and it's running much happier now, plugs are perfect.
Engine is now completely standard, even got a standard airbox for it. Runs awesome, feels very smooth thanks to the balanced crank, much more so than the tohatsu at low RPM.
More props and more testing, got a 2 blade brass prop, 10 3/8 x 17p. Unfortunately first time out it spun the hub... so it will be getting sent off to get a new hub.
Best prop is still the 15G yamaha, now getting a consistent 50 knots with it. Really nice prop, although I'd still like a 16G for a bit more top end, the 70hp has no problem spinning up the 15p. Weather hasn't been too great but got out to little Bernera with it.
Also a few more props to try,
18p merc cleaver (another one)
a cut 15G yamaha, meant to be closer to 16p
And this funky looking thing, 9 3/4 x 17p 4 blade Ron Hill