The keyboard ......
a) get one of the kbd-units sold with old Mirkonik-towers for the A1200. This will turn a A1200-kbd into a AT(=PS/2) kbd, and should
also work with an A600-kbd. But beware those things aren't perfect.
b) Rip the controller out of a PS/2-kbd (quite easy with a Cherry-kbd). Search an A500/A2000-kbd that still had a real PCB
instead of just a foil. Cut the Num-block of it (sacrilege, better use a broken one ;-) ). Follow the traces on the PS/2-kbd
and rewire the A500-kbd to match it.
c) Get a newer A500/A4000-kbd with a foil and a smaller PCB. Connect the A600-kbd to that PCB. Build one of the
microcontroller-based "Axxx gets PC-KBD"-projects found on the Aminet, and reprogramm it to work the other wa round.