seems a few on Amazon / Ebay
cassette or freewheel ?
Shop Bike Conversion Kit E-S Eelectric Bicycle 36V 250W 500W Rear Wheel Hub Motor 20" 24" 26" 27.5" 28" 29" 700C With Lcd Display,Dual Mode(36V/350W,24Inch). Free delivery and returns on all eligible orders.
www.amazon.co.uk
Shop Electric Bicycle Wheel Kit 20-29"/700C Electric Bike Conversion Kit 36V 250W E-Bike Brushless Powerful Hub Motor Waterproof W/LCD Display. Free delivery and returns on all eligible orders.
www.amazon.co.uk
The first option has the far better control system as it is a KT with current control and dual voltage , the downside it is listed as illegal 350w. Contacting the vendor as to the actual etching/marking on the hub to confirm, just a shame it doesn't say 250w in the listing.
The second option is legal 250w but uses crappy speed control.
For a tadpole I would opt for a higher geared internal hub so a 201rpm AKM 100CST 250W 250W, this will mean building one's own wheel. The lower wound motor will enable more torque and better hill and load hauling power wether at 36 v or 48v, at 48v one will get 33% more torque and speed if needed. Though one today would set the speed for 25km/h with the legality issues now being picked up on by plod.
Add a nice KT 9 mosfet 22a controller, at 36v the motor is OK at that current. then if one wanted to go 48v limit the current to 17/18a to save stripping the nylon gears.
Other wise I would opt for the mid drive.