Sitting here in Canada, I have to admit ignorance of the Australian vintage tyre market.
Based on a quick search, there seem to be some sellers in Australia, see
http://www.hcvc.com.au/forum/salewan...es-or-7-00-x20
http://www.antiquetyres.com.au/20700-0
http://www.oldworldlamps.net/tyres_20_inch_16.html
They certainly exist in USA at Coker Tire among others
https://www.cokertire.com/tires/styl...m_diameter=120
Keep in mind that you may not need to limit your search to 8 ply or PR (ply rated as opposed to real plies) tyres, you might get away with 6PR provided they have enough weight capacity and you should be able to use 10PR (or more). If using 10 PR, see if you can get hold of load/inflation tables for the tires. You should be able to run a lower than "rated" pressure as you are also likely carrying a lower than rated load per tyre but without the manufacturer's tables it is hard to guess how much the pressure can be reduced safely. It would likely also work to go up to 750-20 size if that's more readily available. There is a chance the extra width might rub somewhere but my guess is that the chance is small and I doubt the extra 7% width would be obvious.
Looking at your photos, unless there is an illusion happening, I would want to check the condition of the ring in the photo with both red and white circles as it looks as if it might be bent enough to not seat as well as desired. But - if the tyre is just pushed away from the ring, all might be OK.