G'day Joel,
I fear that BB has given you the straight dope. This is a quite complex component, which would cost way more than one mower is worth to reproduce. 3D printing? All very well in the execution, if a suitable feedstock material is available, but the 3D design/modelling is very far from a trivial exercise.
The last available spare part [and that was a rather crude aftermarket repro, made from stacked steel stampings] we at ODK know of went in 2014:
THREAD The last spares that SB/Rover supplied were an aluminium version, but not many Diplomats had those retrofitted, even failure-prone as the nylon gears were. You'd have to be outrageously lucky to find a mower with one of those.
My best advice would be, for you to be on the lookout for a cheapish, rebuildable Scott Bonnar Model 45, on FleaBay, Gumtree etc. Your chances are pretty good in Adelaide; certainly better than the Eastern States, where reel mowers have never been as popular.
You can get a good idea of the specific problems to look out for, by exploring some of the past ODK SB45 repair/restoration discussions - there's not much that hasn't been encountered here.
Awful example
HERE.
