As has been stated above, there have been many threads about these wheel cylinders before, and they are readily available, with cheap postage. For example:
Rear Cylinder and
Front Cylinder. Like you, I live in a regional area, and I can't
drive to Sydney and back for that postage cost, and my local brake parts supplier (ie small/rural, only sells one of those every 10 years) can't match that total price.
As for modifying your backing plate to accept a modern replacement. I would only consider doing that if original design parts were completely unobtainable from any source. I have worked on vehicles that have had components sourced from a mish-mash of other sources, and there is nothing worse.
Have some consideration for the restorer who is going to be working on your vehicle in 25 years time as he (or she) tries to work out what exactly they have before him, and then tries to put together a 3 Tonne Iveco brake shoe with a Case-IH wheel cylinder filled with Isuzu Van cups and 90-97 Landcruiser boots and metric pipe unions on a 1/4" brake line. You really don't appreciate the accursed devilry involved until you encounter it
(of course, on a Saturday after all the shops have closed!
). Consider too, that what you choose today as a modern replacement may not be widely available in the future, while the vintage original still will be.
The original parts
are available and reasonably cheaply, and everyone else will know what parts are on the vehicle and where to get them.