If you were to pull the cover off that transmission you would see the part the shift cable moves inside.
I'm not sure how to describe it but the cable rotates a drum inside.
Anyway, my friend had an SOS and with the cover off I was easily able to shift it through all the gears with a big screwdriver.
At the time I thought I thought it would not be hard to make a bushing and shaft that goes through the cover and shift it with a small external handle .
The attached photo shows another man's contrivance for doing that. His is terribly crude but would be effective.
I think a machine shop could pretty easily make something that was better engineered than that.
As to switching the transmission to a gear type, I have been involved in a couple of those swaps and they were pretty straightforward. First one was in a 1959 871. It was not complicated.
I had an SOS in a later 3 cylinder 4000 and though it functioned perfectly I disliked it - a lot!
So I swapped the entire transmission (and rear end)
to an 8 speed from another tractor.
You being from AU might not have access to used parts like we do here. So maybe consider the external shifter idea. That would allow you to use the tractor while you search for the right parts. If/when you found them buy another cover to replace the one you modified.
Sorry this got so long.