ive looked into this before when i used to have minis and my old fiat 126

i'll id need to do is get a blanking plate for the gearbox and have the cog bit from the bike engine bolt to the sticky out bit in the sj gearbox.... this means i wouldnt be able to change gear of the bike gearbox, id be using sj box ratio still but it means id be able to change bike gear when its parked up.... so leave it in bike gearbox 1st gear for acceleration or bike 5th gear for motorway crusing. this is also how they mount an electric motor for EV cars, big 30 hp motor to a car gearbox mounted the same way.
as far as i know is what i just rambled on about it a way of doing it, its just a case of fitting the box to it and mounting it.