A lot of coaster brakes have a dust cap under the brake arm, that rides on the outside of the hub. You can try this method. Remove the brake arm and see if the dust cap has a D shaped hole that mates with the axle cone nut. If so, remove the dust cap (finger removeable or use a pocketknife). See if the cog will slip over the hub where the dustcap was. If not quite, you can file or lathe the hole a little bigger to fit. The dust cap can be flattened or filed down on its lip, to compress against the cog. (grease!)
This has worked for me for over half a year! (using common single-speed or 6 speed chains. 415 doesn't fit. )