You really need to find some specs, or start reverse engineering things a bit, like how many wires does the encoder have, and can you find any details about the drive that powers the motor?

I'm assuming this is a Fadal, in which case I'd ask over on the practical machinist forum.
It might also be worthwhile having a chat with British Encoders (http://encoder.co.uk), as they might know what you need, or be able to repair or build something that matches.