Pretty sure that’s not a BMW. It’s a flathead, so not an R75, and the flathead BMW (the R12) had a different frame. I think it’s a Chang Jiang CJ750:
Chang Jiang Motorcycles | Chiang Jiang 750 | Motorcyclist (motorcyclistonline.com)
It could be a BMW R71, but those weren’t used in the war, are rare, and are valuable ($30,000 and up).