Right. As already mentioned, the main use for this is not the mere keyboard adaptation (with ADB peripherals sort-of-readily available) but to learn things about the ADB protocol. That would imply examining the original code in order to get the required specs of the protocol, and then creating from the scratch some new code (not necessarily for this MCU) with similar functionality. Is that OK?

The 68HC11 being a derivative of the 6800 family, like the 6502 I'm used to, makes the task of interpreting the original code, by no means a trivial task, but at least feasible.