I believe they are often modern Russian Ural bikes with a suitable paint job. I owned one a few years ago (without the paint). Despite its reputation it was totally reliable but incredibly heavy, the frame seemed to be made of parts left over from building the Eiffel Tower. Or the Brooklyn Bridge maybe. They are now only available here as sidecar rigs, though. The dealer told me he sells quite few with right-hand sidecar, these can't be road-registered here where we go on the left don't you know! but they are bought by war re-enactors and painted up. They keep the machine gun on the sidecar but it's demilitarized.
By the way the bike used by Steve McQueen in "the Great Escape" was a British Triumph TR6 with Nazi type paint job, no other disguise. Totally unlikely!