Upon closer inspection of the crankshaft removed from the engine, it was found that the crankshaft is the correct long stroke 54mm component, identifying the engine as a YD-A, the same as my bike's frame.
When we consider the facts such as:-
1) my bike once belonged to Sonny Angel, where the engine came from,
2) the engine upgrade appeared to be the work of Sonny Angel, who had the means to do so.
3) the engine bears the "O" mark painted on it as Masuko's machine and matches contemporary photo's,
4) the bike (frame YD-A YD21) was definitely the Catalina Prototype and therefore the Asama Winner which was shipped to the USA ahead of the following 4 machines and differs in the design of the footpeg fixings..
The facts are quite convincing that this engine is indeed the original engine from my bike and that this bike is the actual Asama Winning machine ridden by Osamu Masuko.
The crank is stamped P, indicating 54 x 54 mm bore x stroke.