It depends how well you want it to work...
Anything of a length to give you an appropriate ride height and holes that the mount bolts fit through will 'work' after a fashion, but probably won't be 'optimal'. If you don't mind losing a bit of rear height, an ST1100 rear shock will 'fit'. It'll be a shade stiff (ST is a little heavier) but if you find an aftermarket one you could get it revalved -- but then you're looking at time and money again.
The problem is that having a monoshock in twinshock position is pretty rare, so there aren't many options out there compared to other systems.