Can't find any numbers on the engine.Yes it is a JAP engine.Here is some pictures of the engine.Not sure if this will help or not.
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There is some numbers cast into the cylinder they are,20362 on one side and o4 on the other.
Also found on the exhaust HB20525.Also under the fuel tank found a stamp the had DJH 1955 there. Perhaps a date stamp maybe?

Here for a good time,not a long time.