I too have Miller electrics on my 1959 model. I needed a new distributor cap about 35 years ago (on the ’59 model I then had) which I got direct from Miller who sent it along with a wiring diagram which I still have (the distributor cap went with machine when I sold it). Their documents refer to Douglas Dragonfly and this has been crossed out by hand and Scott written in. This suggests that the Douglas OC might be an option.
If of any use, I can take photos and email.
Note: on my last ride it started to rain and there were lots of very powerfull (and pretty) sparks over the top of the distributor cap when it would not start in the pub carpark – I’ve obviously got good electrics (its been converted to 12v), but the distributor is not a good design.