These were based on the Blackstone design, but the 920 was several major model revisions from the Blackstone original.
basic Chronology-
500 series is pretty much Blackstone design.
600 series almost the same but with pressure-switch-operated brake for neutral drain.
700 series has electric pump and plastic outer tub
800 series - I don't remember the key changes over the 700. Help anyone?
900 series - I think this was the cheapened up version. Flimsy plastic top and lid, short agitator shaft, spin runs in a top bearing (with triple lip seal) instead of a bush ( with carbon seal). (So not the same as Blackstone.)

Matt I have never had one of these apart (have done a 720 many years ago) but the basics of your question would be: if the old seal has worn grooves in the shaft, you need a new shaft. I'd expect that will be hard to find but you never know your luck.

the inner agitator shaft seals - are they simple O-rings? If you can get an old one out, you may find them available at any bearing supplier. Just take one of the old ones in (cleaned up) and they will match it. It they don't have it, ask where they recommend you try. I have often finally found some hard to get part by asking that question, and sometimes following a chain of "Who do you reckon might have it?" before hitting the jackpot.

If you can't find a new shaft, you can remove the old one and get it machined. Any decent machine shop will be able to clean up the worn shaft, then either (1) fit a stainless steel sleeve to cover the worn area (I think they are called Speedy sleeves?) or (2) add weld to build up the worn bit, then machine the weld in a lathe to restore a nice even surface.

What part of Melbourne are you? I might be able to suggest somewhere. I live out in the sticks west of Geelong now but used to live in Melbourne.