Easiest is to get a coaster brake hub sprocket, 21 or 22 teeth, from your LBS. I ordered one from either bikepartsusa.com or Niagara cycle works for like $2.50, plus shipping of course; not worth ordering it by itself, but if you need a few things, it can be worthwhile.
Are those 3/32"? I don't see those on Niagara, unless I'm not looking in the right place.