Over a couple of hours today I have worked out the CAD details for the timing belts and pulleys. Now teeth and proper pitch lines are modeled in to all parts. I am not 100% sure that the geometry of the teeth on the pulleys is correct but it should be 3D printable and work at a basic level.
The belt is a 48 tooth T2.5 timing belt (already in the post from an aliexpress supplier) and the pulleys are both 20 teeth. There is of scope for increasing size of the pulley on the arms for a reduction drive but this will likely require a longer belt.
If the 3D printed pulleys work out then in the future I will put together a guide for modeling the teeth in a CAD program as I could not readily find advice of this nature.