It may be easier just to fit different swing arms ?
I've got a pair, although I live right down in Leicestershire !
If you want them you can come and collect, free of charge !
I'll look at the bushes on mine in the daylight to see if I can work out how to remove them.
I admire your determination in doing the gearbox !
I did a mini one about 20 years ago and thought NEVER AGAIN, although they are far, far more difficult compared to the nice Fiat one.