There's a guy selling those cast-wheel kits on UK Ebay:
http://www.ebay.co.uk/itm/New-design-Magnesium-Alloy-rear-wheel-electric-bike-kit-motor-36v-500w-48v-500w-/131111526815?pt=UK_Bikes_GL&hash=item1e86db219f
Xofo don't want to sell single kits. I asked Colin Sheng, the proprietor, and he said trade customers only, which means a minimum quantity.
Xofo make several different motors. It was the 250w one with the inbuilt torque sensor that had problems with the wire burning, but that was a long time ago, and the motors are still in production being used in reputable OEM bikes, so I'm pretty sure that that problem has been fixed. The kit in the photo above is not the torque sensor motor, and neither is it 250w, so the burning wires is not directly relevant. i don't know about the 20" 250w version.
I had a short test ride on an OEM bike with that motor. It had the nicest power algorithm that I've ever tried - better than Bosch and Panasonic until I came to a hill. It didn't have a lot of torque for hill climbing and on steep hills, the torque sensor springs bottom out, which causes a sort of knocking, which you feel on the pedals. It's a bit like having loose cranks on a square taper BB. It's not a problem once you realise what's happening, but a bit disconcerting the first time you experience it. In summary, it's a very nice system if you're light-weight and/or you don't have steep hills.
Xofo also make a clone of the 500w Bafang CST, which has a cassette spline. It runs happily with 30 amps at 36v, so plenty of climbing power. It feels/sounds the same as the Bafang - smooth and fairly quiet - but it has a slightly lower RPM. It tops out at about 20mph on the road at 36v.
Finally, they've just released some nice looking crank-drives. I've not seen any tests yet, but I would expect similar performance to the Bafang BBS01. The Xofo looks more stylish.
