Up thru last year, I think Bafang only made one size motor that fits fat bikes. Does it look like this?This one is 350 watts and 36volts. I bought it for a fatbike, and I will install it when Spring comes. The controller I have is very small, and rated at 17A.
For more power with an existing motor, people look at bigger battery, a higher voltage battery, and a controller that can take advantage of more voltage and/or battery size.
These guys sell a controller for the Sonders fatbike that uses a Bafang rear motor, which I think is the same motor I have. The connectors look the same as mine. It could be compatible with your motor. After reading your plight, I think I will order one. You might contact the on their support email to see if it could fit your bike.
The bigger controller only works if you have enough battery to push the higher current. If your battery is too small, you're stuck.
Storm Hot Rod 25 amp controller BIG 36-60v - Luna Cycle
