here is a threaded on skiptooth cog to fit a BSA freewheel threaded rear hub. https://www.ebay.com/itm/385293993992?var=653182991494 . they may be others available from another source. be aware that 1 inch pitch chains may require a different rivet tool, although i found that a park CT-3 works on a diamond skiptooth chain, but not maybe other brands or block chains. park CT-2 will do the trick though. old skiptooth cogs that mount on new departure, JC higgins or Musselman or other pre war hubs don't fit on modern rear hubs. it's a rabbit hole. and if you decide to build up a rear wheel with one of those old hubs with a sprocket already mounted, good luck trying to remove the cog to build the wheel. i ended up finding a hub driver as a part without a cog mounted, and temp replaced the existing one, then after the wheel was built, re-installed the original hub driver with it's hard to remove cog. the only reason i went to the trouble was for 1930's wastyn and 1940's sieber track bikes.