Where are you based? I (and others on this forum) have used a company local to me in Dorking called Morgan with good results. They certainly know their way around 50's Bosch, Lucas & Marelli, and I don't suppose the 30's item would be much different in design (my 'Aprilia's 37 Marelli / Bosch dynamo certainly isn't).
http://www.jmorganandson.co.uk/