New Miller rotor arms AND DISTRIBUTOR CAPS are available from Kip Motor Company in Dallas Texas. I got them made for my Dragonfly some years ago and they are a superb copy but because they use modern resins rather than Bakerlite so they are slightly heavier. I have tried on many occasions to get the Douglas spares scheme and the Scott one to jointly buy a few because they are very expensive but there are significant discounts for quantities.
Their website is https://www.kipmotor.com/catalogs.htm They do not show the rotor arm in their catalogue but they do have them. Their response to emails is very slow so better to phone.