I researched this a lot when planning to install the BBSHD 1000 on my KHS 500. Take a look at EMPowered Cycles. They sell a PF41 adapter for the motor, the price I saw was $74.95. There may be others who make an adapter as well. One important consideration is the chainstay clearance on the sprocket side, it may be to your advantadge to try the 120 mm version of the Bafang, you can adjust the sprocket clearance with shims that should be available at your local bike shop. Hope this helps.
Matt was the first to do the adapters and conversions. Other companies used his information and web pages to sort the possibilities. Shims will be cheaper, in my experience, from the motor source. The only problem with a conversion using shims to create clearance, is the possibility of the cranks not being equidistant from the center of the frame. Irritating to some and not a bother to others. Those that will pedal a lot will be bothered more than high throttle users. In my experience.