On the basis of only changing one thing at a time, have reinstated the fencepost tower and refixed the genny pointing West. It's now using the 10ft 6in prop, and we'll see how that does before thinking about changing to a swivelling mounting. The forecast shows reasonable winds - but from the East, which is typical. However, if I wait long enough.....

Local engineering shop has done a very good job on the adaptor.
The original fitting (bottom left) is soft alloy, they recreated it in steel, with a big bolt , and counterbored to make a location for the hub prong. There's a transverse hole to take a spring pin (though I might use a bolt instead) to transmit the torque, and the nut at the end is held on with a split pin.
The hub had a tapered hole down the centre and a UNC or UNF thread at the end, they cut the thread off, bored the hole out and fitted a length of steel tube in there with a flat on the prong so it only slides into the adaptor one way.

After some gales, I hauled it down for a check and discovered that the 3 wooden blades were all split along their length. Oh dear. So I sent off to CMS Magnetics of Plano, Texas, for a set of their 10ft 6in reinforced fibreglass blades plus hub.

Here are the blades, which all balance at the same point along their length. However, one weighs 3020 grams, another 3042 grams and the heaviest comes in at 3055 grams.

My friendly local engineering shop is making an adaptor to mate the very substantial 3/16 inch steel hub to my 1KW stepper motor. Once I've mounted the blades, I'll see if I have unacceptable vibration when they are spinning. If so, I'll try adding weight to the appropriate blade at the hub end, and if that doesn't work I'll sand down the heavier ones (I need to clean up and sharpen the blunt edges and reshape the tips anyway).