Nice Bike!! How do you like the coaster brake setup? is it difficult to modify the brake itself or is it just a matter of taking it off to mount the sprocket?
It works great, the arm needed rebent to clear the bolt heads, I seen one where the bolts were countersunk and solved that problem, and the whole needed enlarged to clear the dust boot, I just ground it out with a burr and a die grinder.
I enlarged my sprocket hole with a metal rasp that fits into an electric drill. took a while but it worked quite well. Hardest thing for me was bending the brake rod without a vise. It can be done but take a while and some patience.