Why dont you turn the clutch inboard to make the jackshaft narrower?
Would the motor turn the other way and not run the bike in reverse?
It looks like the clutch hub is machined like that, kinda pear shape, are there torque specs for the nut (or bolt) that holds it to the shaft. I don't think the key is susposed to do more than act as a safty,,and the hold down nut/bolt does the actual work secureing it,kind of like the flywheel and key on top of a 4 stroke lawnmower.
As long as the hub still fits the shaft snugly not woobly,I would think another key and tighten the nut/bolt good and tight.(or to spec's if you got them)