Boxer-Engine Motorcycles by Ratier and Zundapp.
The boxer layout, with the crankshaft longitudinal and opposed cylinders, was developed by BMW Engineer Max Friz, and resulted in the 1923 BMW R32 the first BMW motorcycle (yes, Virginia, BMW made motorcycles five years before it produced cars). This engine layout always struck me as being utterly logical. The cylinders project sideways into the wind and have good primary balance, and transmission to a shaft final drive is relatively straight forward (or backward), eliminating any need for a bothersome chain or belt. Other manufacturers, most notably Honda, recognized the singular virtues of the boxer/shaft layout and produced motorcycles so equipped.
A surprising number of manufacturers produced transverse twin motorcycles in the boxer (flat-twin) or V-twin format with shaft final drives. CEMEC of France made 750cc side-valve twins from the late 1940s into the 1950s. In China, Chang Jiang still makes boxer twins, based on pre-war BMWs. Switzerland's Condor produced the 750cc SV EC850. Cossack, USSR, produced the Ural, which later became a manufacturer in its own name. Douglas, in England, produced in 1934 the 500cc Endeavour. France's Gnome & Rhôme made in the 1930s the 499cc OHV CV2. Germany's IFA produced a 350cc two-stroke flat twin from 1954-1960, when it became MZ's BK350 model. IMN, in Italy, made a 200cc(!) OHV Rocket in the 1950s. Of course, Indian, in the U.S., made in 1941 the Model 841 for the U.S. Armya 750cc SV transverse V-twin. Harley-Davidson, like Indian, produced the flat-twin XA for the U.S. Army. Even Italy's famous scooter-maker, Lambretta, got in the business in 1951, producing the 250cc V-twin SOHC Racer. Japan's Lilac was famous for producing shaft-drive transverse-engined motorcycles both as V-twins and flat twins, including the 300cc V-twin OHC MF39 and, later, the Marusho. Italy's Moto Guzzi has been producing transverse V-twins with shaft drive since the model V7 in 1967, and still does to this day. Ratier, in France, made both SV and OHV twins after World War II. In Switzerland, Universal made a 580cc flat twin starting in 1946 in both OHV and SV versions. Germany's Victoria made the V35 Burgmeister 250cc transverse V-twin with shaft drive starting in 1951. Zundapp, in Germany, was as famous as BMW for a time producing SV and OHV twins in a variety of configurations, most famously the 600cc postwar KS601 (see below). And, believe it or not, there were other manufacturers, including Velocette, ABC, Capriolo, FN, Ratier, Sarolea, Hoffman, Delco, Economic, Chang-Jiang, Crosley, Windhoff, BFC, Dnepr, Dong, Imperia, Wooler, Moto Fran Caise, BIM, Yangzte, Puch, and Jawa! Last, but not least, would you believe a 170cc 1962 Honda M85 scooter with boxer twin and automatic transmission? Believe it!
Other manufacturers weren't satisfied with two cylinders in their shaft-drive, transverse-engined motorcycles. Honda's Gold Wing, with four to eight cylinders, is very well known today. England's legendary Brough Golden Dream is less well known, but incredible nonetheless. Famed airplane designer Glenn Curtiss built in 1907(!) a shaft-driven V8 that went 136 miles per hour. The next V8 with shaft drive didn't appear until 1994, with Morbidelli's 850cc V8 from Italy.
Germany's Zundapp produced a variety of motorcycles with opposed cylinders, in both two and four cylinder arrangements. The first of these was the 400cc K400 of 1932, which was followed by the K500, S500, KS500 (1935), KS600 (1936), KS750 (a wartime cycle), and the four-cylinder(!) K600 and K800. The postwar 600cc KS601 (photo below) was an excellent motorcycle produced from 1951 to 1958, but could not compete in sales with the better known BMW models. Today the KS601, popularly known as the "Green Elephant" has a strong but small following and is often restored.
Germany's Zundapp KS Boxers
The drawing below is of the amazing prewar four-cylinder Zundapp K800. The British Brough Superior Golden Dream, another four cylinder opposed engine motorcycle, did not make it into production.