Give Lawrence a call at Mendelssohn Pilot Supplies in Scotland, tel 0044131 447 7777. He is a real helpful guy.
The connectors for the back of the Narco are still available from RS components, called a molex and the wiring is pretty straightforward to the encoder, any competent person who works with electronics would be fit to manufacturer the lead, but Mendelssohn's should be able to supply it "plug & play" compliant so to speak.
Hope this helps.
(PS) I have the schematic wiring diagrams for the AT150 to encoder should you need it, drop me a PM if so.