If you have a look here:
This guy has got the original working through a hacked usb keyboard.
Edit: I think it might be one of those listed in the ebay link above.
It's entirely do-able.
For my initial Pi project, I've taken an old Amstrad CPC 6128 and hacked/soldered the connectors to a usb interface ripped out of a flexi keyboard (as did the Spectrum guy above). I used xev and xmodmap to create new mappings. Luckily for me the Amstrad uses a 8 x 10 array which fitted nicely.
I count this as the most difficult thing about my Pi 6128 case mod. Everything else is just gluing and cutting.