Been keeping an eye on this out of curiosity.
Have you had any luck with your sprocket search?
If you can't ind one, you may want to have one built. If it is using the square spreader apron chain,
a sprocket can be made from a piece of plain old mild steel.
Determine the radius of the outer end of the teeth, layout a circle,
then divide the circumference for the proper number of holes,
drill out the holes to be the reliefs between the teeth portions, ( it's been a while but I think I used a 5/8's drill for mine)
then cut out the sprocket cutting just outside the center of the holes,
square up the slightly more then 1/2 wholes with a grinder.
It will not last as long as the old one did and is not as strong as the old casting.
But I doubt that you are using the spreader as much as it was designed to be used.
If you are a machine shop can manufacture your gear easily.