Bonhams sold a 1902 De Dietrich car, and mentioned that the Dietrichs' were wealthy landowners in Alsace who dabbled in early car manufacture and racing.
They likely are still there, maybe any enquiries about motorcycles should go to them ?
They, of anyone, would likely have preserved some details and maybe even pics
btw, the car sold for ~$1,700,000 so any (surviving) motorcycle might be somewhat rare ?