@linear good because I was about to ask that :P from a bit of research there's https://www.bigmessowires.com/usb-wombat/ for ADB which is very bulky and expensive,
there's also an arduino implementation for ADB which looks much nicer: https://github.com/Difegue/Chaotic-Realm/tree/master/adbduino (needs an extra board to connect it to the mini-din)
https://github.com/Harvie/ps2dev seems to have ps/2 emulation done

pcb would be quite simple, connecting both the connectors to an arduino, basically