Chris
Honestly this is a very old set up, you may be able to find a used one somewhere, I used to have a Sodi Kart, shifter kart that had that exact system, and yes, you’re right it was also utilized on Trackmagic karts.
Here’s a link to that old caliper system. Hope that helps, I honestly would recommend just buying a new more modern caliper and bracket system.
http://www.fastech-racing.com/brembo-rear-caliper-20.5161.71.html
And here’s a lager one
http://www.fastech-racing.com/brembo-20.6950.10-rear-caliper.html
I think I only kept the front calipers and lines when I sold that Sodi kart.