Is it a fixed gear cog (1.37 x 24 tpi threading), or is it a threaded cog for a coaster brake? Threaded coaster brake cogs are usually a variety of sizes and usually won't fit a track style hub. If the threading is smaller than that you might be able to have a machine shop thread a generic freewheel rear hub, which would allow you to use the cog and lock ring from a coaster brake.