Kevin,
The early Dragonflies had Douglas wheels and the rear sprocket was part of the brakedrum. Later Dragonflies had hubs made by the British Hub Company - these had a bolt on sprocket that was machined away to fit the register on the brakedrum. If you can get the machining done, sprocket blanks can usually be obtained from bearing stockists - they know them as 'Plate Wheels' - the standard Dragonfly rear sprocket has 51 teeth. If you are attempting to repair an early sprocket, get the original sprocket machined down and a new ring welded on - leave the 5 legs of the old sprocket on the drum and weld to them - don't weld direct to the drum as that will distort the drum and adversely affect the braking.
Hope this helps,
Regards,
Eddie.