There are various teased products out there, from replacement keyboards (like the kipper2k), to usb converters like the keyrah, but most of it seems like vaporware or at best is almost impossible to get a hold of.
For those of us who are interested in building emulators or FPGA based systems in a new A1200 case from A1200.net, there aren't many options for keyboards out there.
That said, DIY mechanical keyboards are relatively easy to do, but most of us don't have the knowledge to create a PCB; especially if it needs to conform to an existing design that a lot of us don't have access to directly measure.
How simple would it be to convert your existing PCB design to create something with a simpler column/row matrix that can easily be connected to a cheap micro controller flashed with one of the common keyboard firmwares, like KMK or QMK?
As an aside, what profile key caps did you use for the keyboard you have pictures of in this repository? It looks like DSA or something like that. I would assume not all key cap profiles will work for this.