There are two ways to tackle this, either "downstream" of the controller or "upstream".
In the downstream method you figure out what protocol the controller is putting out (XT, AT, some terminal protocol, etc.) and convert that to USB. Soarer's converter works this way, for example. With this method the original keyboard controller is still being used, and must be functional. The catch is that whatever protocol the keyboard speaks must be "on the list" of protocols understood by whatever converter you plan to use. If the keyboard speaks some weird dialect not in the converter code, you're pretty much stuck. Unless.... if you can figure it out, Hasu might be able to help you add it to his
conversion code.
An upstream conversion would be to replace the controller. You'd use a multimeter to figure out and diagram the keyboard matrix (not nearly as hard as it sounds) and then wire the matrix into a Teensy or other controller. Using QMK or TMK you'd probably find an existing keyboard file which is pretty close, modify it to match your diagram of the keyboard matrix, then flash the Teensy with your file. With this method the original controller is bypassed, and can even be removed.
I realize none of this is any direct help but I hope you may get some ideas.
