850T clutch arm

Somehow the lock nut has fallen off the back or the clutch arm on my 850T. It’s a right squeeze trying to get it back on, especially as there seems to be no thread! Anybody got any ideas how to put it on without taking the engine out? Clutch isn’t working very well without it…

I assume it is the same set up as the slightly later 70’s models, if you remove the pin you can drop the whole arm out and then screw a new nut onto the adjuster.
If the pin holding the arm in place on the back of the gearbox is siezed, be very careful trying to tap it out as the lugs on the back of the gearbox are very fragile (Don’t ask how I know)
Worst case, it would need the swing arm dropping out to get a clear view of the job.