You just buy a length of chain a little longer than you think you need and a master (removable link).
You then drape the chain over the sprockets and mark it to the length needed.
Remove the extra links and join the chain with a new master link.
I assume you already have a rotary autopilot. You can remove the master link and measure the chain.
If it is a new installation just order more chain than is needed and do as I have written above.
Kimberlite Amel Super Maramu #376
Is there any that knows the length of the chain for the autopilot (rotary drive)
Niklas SY Nipe SM2K#333