Last time I looked there was one option on the Luna site, a BBSHD which isn't a bad choice, and no 48V or 52V batteries. You might be interested in the tutorial on ebikesca. Should you decide on a mid-drive, make sure you measure the width of the bottom bracket (probably 100 mm on a Fatboy, but that's a guess). Also, be aware that depending on how the chainstays flare out, the motor may not "nestle" against the frame. It would be advisable to locate someone who has accomplished the conversion. Try endless sphere.