On bike A, I had a front wheel motor that worked, but the bearings that support it were very loose (worn, I think); probably need to be replaced. I don't know if that's actually possible.
On bike B, I had a motor that worked only in fits and starts, but good bearings.
So, I took the motor from A, the bearings from B, and assembled a good front wheel, put it on bike A, and wired it up. The above sentence summarises a few hours of work (some of it mechanical, some electrical) ... but once it was done, it worked, and I now have a full-size bike (26 inch wheel) that works very nicely (I also adjusted the return spring on the front brake).
For bike B, I now have a front wheel on wobbly bearings, and a motor that doesn't work. But I have a front wheel and motor coming here on a slow boat from China. When it arrives, I'll let you know, but that should give me a working bike.
And I still have a working 20-inch wheel electric bike, which will become the first reserve, in case anythng happens to bike A.
I'm pretty pleased about this!