T2R20A, T2R20A-A, T2R20A-X are all the same engine with minor differences. Dipstick moved to the opposite side etc. The engine tag for the US ones (YM240) says 24 hp, that's the naked engine on a test stand per the US rating method, so the customer can compare this against other US tractors advertised by that method.
YM2000 with the same engine has a tag that says 20 hp, that's the PTO output figure that the customer in Japan needs to know, to match his tractor to a specific rototiller.
YM240 is the same tractor as YM2000 with very minor differences to conform to American expectations - pull the throttle lever to slow down etc.