A couple other things to do to get all notes playing properly.
The vinyl bus bars get worn out and you'll probably see groves worn into them by the contact wires from the keys. You can push the contact wire array for a key slightly right or left so it hits fresh contact bar vinyl OR you can rotate the contact bus bars to make the silver contact wires hit fresh vinyl.
Playing notes a lot can help, too.
Conductive vinyl was used to eliminate the clicks & pops of a hard silver contact bus bar. The tibias have silver contact bus bars but the keying electronics have a soft turn on/off for the tibia voices.
Watch ebay as stated earlier. I bought an octave of spare keys and brass key springs -- just in case -- for a Conn 651. All Conn keys were the same. I now made a midi interface that can mount on top of a Conn keyboard. I did it mainly to play orchestra bells (also often found on ebay for about $200-$300 US.) Having the spare keys on my workbench really helped in designing the magnetic contacts for the interface.
Digital Pipes
http://pstos.org/instruments/wa/seattle/lajko.htm