My first idea was to use daughter.1's old (20 years or so) bike; first as a truing stand, and then to convert to electrical operation. That fell at the first hurdle - the aperture in the forks is too narrow to take the wheel axle. I'm guessing that this is because it's A) old and B) US brand (Specialized).
So, plan B - I have an Everest folding bike that I could use. That worked fine as a truing stand (the wheel isn't completely trued, I got bored with it, and I'll continue later). Also, I wasn't certain that the new motor would work with the new controller (from different vendors) and the throttle (left over from another bike). So while the wheel was still not entirely true, I connected it up and tried the throttle.
So I had a think. First of all, there's a switch connector, useful if you're going to have a key to switch on. I don't much see the point of that, either the bike is locked with a substantial lock to something solid, or else it isn't. There are bike thieves around, certainly, but I doubt if there are bike battery thieves. So I just shorted that connector.
So I had a look at the throttle, and the connectors are in a different order. I reshuffled them, and oh joy! The wheel spun!
So I made all the connectors a bit more permanent, using screwed connectors, which means I'll be able to take them apart if I need to, but they're firmer than push-fit, which is what it expected to use. And I checked out the battery voltmeter connection, that worked fine.
But the problem with using this folder as an electrical conversion, is the brakes. They're disk, front and back, and although that's usually a fine idea, in this case it isn't, as I have no way to Add disk brakes to the motorised wheel.
Could I put in caliper brakes on the front? No - the trouble is, the front fork doesn't have the necessary holes to take them.
So, what to do?
I could buy another folding bike and do a conversion on that, but surely I have too many bikes already?
I could run the Everest without front brakes ... I don't think so!
I could replace the front fork of the Everest. But that's going to be at least £40 for the new fork, plus loads of work.
So, I think I'll do nothing. I'll finish truing the wheel, and then put it, and the controller, into a box, to be used next time a motor stops working. Unless I can think of a way to have the front brake working.
... later ...
I did some research. If I get caliper brakes, then I can get a kind that uses the existing one-hole fitting in the front fork to mount it. It means I use single-pivot instead of double-pivot, which means that the brakes might be slightly less good. The next problem was to find brakes with a reach (distance from bolt to pad) that could get to 90 mm, and the problem with that is that so many people selling brakes don't bother to give that essential measurement. It turns out that what I need is "old-school BMX-type" brakes, so I've made an order on Ebay. £5 plus £3 postage. After I have this installed, I'll try out the stopping power, but the rear brake will still be a disk brake, and I use my rear brake mostly, I only use the front brake when I want to stop really quickly.
Truing continues; the wheel is getting closer to true.
So maybe I can make this Everest into an electric bike after all!