Chains are fine and all, but something of a maintenance nightmare. Quaife makes a lovely transaxle that will mate right up to a fireblade motor, and supposedly others, although I've heard complaints. But damn are they pricey! Transworks mini-diff always looked good on paper, but they stopped making them, and there were less-than-thrilled customers. If you mount directly to a diff, there's going to be a lot of distance between the motor and the axle, and the motor might end up too low.

What else is there out there?