Here are a few old photos, may be of interest. Sorry most have faded with age! Most are of the engine room, so to speak.
1) a Danish Nimbus, parked in a Copenhagen street in 1970. See the exposed valve gear, pressed steel frame and elastic band suspension for the seat. These were made until about 1957.
2) A Belgian made Sarolea Atlantic, seen at the Autoworld show in Brussels a few years ago. The bike was built around 1956 and the company went out of business soon after.
3) A 600 cc Scott two-stroke twin, British made, parked in a London street in 1957. See exposed chain drive to magneto. These were built in various forms, on and off in small numbers, I think until around 1980.