One of my favorites that don't get out often, a little 1hp Stuart Turner WD Flat Twin. Built for the War Department under strict constraints (ST were the only company to meet the asks) from the 1920s until 1945 they replaced the Marconi Douglas engines for front line Wireless sets. So good in their operations 2 sets were taken on an Everest expedition and used unmodified at Everest base camp (about 17,500ft) setting a record that was later beaten by a highly modified Norman S/C.