JAP made an extensive range of V-twin and single cylinder racing engines through the 20s, 30s and 40s in sizes from 250cc to 1260cc. Most of them never returned to production after the Second World War, but the 350cc and 500cc singles continued to the late 1960s, with limited production for some years after that. Google 'Alf Hagon' if you want to see a V-twin in action.
the 'double-single' type v-twin with two single-cylinder top ends was first tried at Brooklands and for World Record purposes by the late Eric Fernihough and subsequently by Noel Pope, who ran one at Daytona and Bonneville among other places, Pope's machines were supercharged but Fernihough used conventional carburettor induction. Fernihough was killed in a crash before his machine set any records, and Pope's machine suffered from severe handling problems caused by the imperfectly-understood streamlined shell and again, didn't achieve anything.
I've never seen a Harley bottom end used for this before! The top ends are the early '5-stud' type, these were often converted to fit the later 4-stud cases - as these ones have been - and seem to be the early left-side inlet type, later ones all had the inlet to the right for speedway racing ( the right side of the mudguard on speedway bikes of the day had a valance to keep flying shale out of the inlet ), although they could be converted. I'm not 100% sure whether any left-side inlet engines had the later enclosed valves, or whether they were all the original 'dog-ear' type
Mongo only pawn in game of life