The wiring diagram I got from Miller in the mid 1960’s was actually for a Douglas Dragonfly with a minor mod for the Brum, hence the suggestion to look at the Douglas source sounds good. If you want a copy of the wiring diagram let me know via an email to private email address as per Yowl.
Whilst looking at the pancake dynamo on Ebay, I noticed that the same seller has a Scott A2S Flying Squirrel aircraft engine for sale. Worth a look.
Item No.160259358327