I was looking for the service manual the other day for the autohelm 4000 and I downloaded it. The part number is "D169" if that helps. I did a quick search on that part number and couldn't come up with anything other than what kvaavik said about Raymarine's solution being upgrade. A search for "autohelm 4000 parts" did turn up this site tho for a custom belt company Polytech Design Inc - Timing Belts & Pulleys
. I couldn't find the part there, but you may try to call them??