The GTÜ classic car service is a public information and archive system for classic vehicles which includes listings for all major and a good many more obscure German motorcycle marques. There are similar pages available for other European brands.
GTÜ Gesellschaft für Technische Überwachung mbH
Vor dem Lauch 25
70567 Stuttgart
Website: GTU
High quality photography of Vintage, Classic and Racing motorcycles. An introduction to his encyclopedic catalogue is available here: