Thanks, Bob D.
Well, the Nuvinci is a bit steep for my budget, though a rugged version is quite appealing. I imagine it might help me to make the best use of the permissible 200W motor output, especially whilst adding pedal power. My fixed ratio HT won't go a lot faster pedalled, as opposed to when not pedalled, but I'm sure some sort of CVT would make all the difference to that situation (?).
Ah, GX35 centrifugal clutch comes in strong at 4000 RPM. Hmm, I hadn't considered that as a factor in my gearing analysis. I'm guessing that peak torque occurs at the same RPM as optimum efficiency, so the 5500 RPM cruising rate is starting to sound more sensible.
Oh, I think I might have seen one of the videos of a motorised bike using the standard gears and derailleur to change ratios. It looked a bit scary though, as the guy on it was doing motorcycle type speeds, and I'm not really that comfortable above 30 km/h myself (~20 MPH).
Well, I've priced out a few different options, and I think it's going to have to be a side mounted setup, with one of those tiny 5:1 gearboxes, chain-driving a 44-tooth (hopefully LH freewheel clutch mounted) HT style sprocket. I just need to find a half-reasonable and affordable freewheel clutch with a left hand thread, and some 14mm, 6-spline drive sprockets to attach to the gearbox. Presently, I'm looking at about $200 in delivery costs alone for those 2 items, but everything else is cheap and close at hand.
T.