I found a site offering a similar service:
https://digilink.com.au/about-us/
They are in North Ryde
There is a phone number on the site it may be worth giving them a call.
I used to work in an industry where we required this type of service regularly and got them done in a place in Mascot but that was 30+ years ago, the chances of them still being in business in this day and age are slim. Add to that the fact I can't remember the company name....
Sorry!
The USB to serial, if you're just going for an adapter which converts there are adapters then there are adapters. Don't cheap out (sorry, I'm trying to remember the brand of the good quality thing I got, I'll find it later) I bought a few before I got one which worked with an Elbex speed camera (former, I played with it for astronomy, amazing camera, amazing zoom (totally different sensor for zoom) and amazing pan and tilt capabilities controlled by xbox controller) but that coaxial signal was always lousy.
You could ask at the company I put a link to, they may do that too for a custom made unit.
Good luck!