KORG SIGMA repair KEYBED INFO HELP REQUIRED

[Rimbard]

Hi all... I hope someone out there can help me with the repair of a Korg Sigma that I have aquired. It is missing the keybed !! ( it was purchaced from a French man and it had a french accordian button type keyboard fitted when he had it)
I am trying to locate a suitable 37 note "normal" keybed to bring it back to life again.
The resistor ladder network for the keys is still present, (just 37 wires and a return) so I guess its a simple switch type keybed would work. It does play all the notes when you short the return wire to any of the 37 wires... so I have to find a keybed for it !!
However I also can find no information on how the aftertouch works, or was implemented in this synth.
Can anyone help? Have photos, information on this.
And any idea where I can find a keybed. (Even a different one that I can modify to fit/work)

Cheers
John