One of the problems with bike engine installs, is that while the gearbox ratios are very similar to car gearbox ratios, there is also a primary reduction between engine and 'box.
Overall, with a conventional ratio diff, you end up with an under-geared set up.
One of the favourite diffs to use is the 3.14:1 ratio Sierra 7" unit that was fitted to one of the old Diesels.
Unfortunately these are also extremely rare nowadays, and with them being so sought after for your sort of project, expect (if you can find one) to pay blood money!

Alternatively, and becoming popular now, is the diff from the Freelander. The ratio of this diff is 3.21:1.

Quaife manufacture ATBs for both models of diff