from what I've seen looking at other build sites the bike transmission is nearly always retained and either a modded car diff (to carry a sprocket) is used. Or a there are units built for the job from Quaife for example.
I think your best bet is to ask this question on:
LocostBuilders
Where there are many BEC head bangers hanging out.
Also check out:
zcars.org.uk they do kits for this sort for thing for Minis they might be able to help you in some way.