8FUN (Bafang) has the majority of the mid-drive DIY conversion activity; their BBS02 & BBSHD are well received and seem to have decent track records. I've used a BBS02 for the last year+ and it's performed perfectly. Look at Luna Cycle for the best prices. electricfatbike.com has more about them than toy'll ever want to read. Biggest consideration (IMO) with an AM (or most full suspension bikes) is where to put the battery.