The "dirt track" bike was a very formulaic thing: the Webb DT fork in a frame of very well prescribed geometry with a single-speed countershaft, either with or without a clutch, depending on racing regulations at the time. All this stuff was designed by Australian Alan Bruce in 1928. His racing bike was taken to England in 1929 by Melbourne Rudge Agent Tommy Rogers and presented to the Rudge factory, who used Bruce's fork, geometry and countershaft for the all-conquering 1930 DT Rudge. The fork was put into production by Webb and sold on all speedway bikes up until the 1960s, and the countershaft and geometry could be copied by anyone, so a 1931 DT JAP was just a JAP engine in someone's DT frame. Martin were big manufacturers, and if I recall correctly these were sold by Commerford's - thus the reference to Floyd Clymer importing Commerford-JAP speedway bikes. The Crocker speedway bikes either used these frames, or copies of them, so while not "Rudge" they were more-or-less 1930 Rudge pattern, as were all speedway bikes. But I don't think short-track speedway was ever really big in the US, at least not as big as it was in the UK and Australia.
I'm not sure what to make of some of the Crockers that come on the market these days...
Leon