Thanks for the info, Greg. I suppose that it could be the product of one of the Tanaka operations other than the one in the U.S., or it might just be old enough that there's no longer any reference to it on the U.S. site.
"VA" is presumably volt-amps... the product of multiplying the voltage (in volts) by the current (in amps). For AC power, if the two are in phase (which happens with purely resistive loads and is called a power factor of 1), that's equal to watts. Some loads (such as motors) force the current to be out-of-phase with the voltage; the alternator may be rated in volt-amps because that's what's limiting it, even if at 400 VA it's not getting as much as 400 W out.
I don't know why it has both 400 VA and 120 W ratings... perhaps the first is for AC output and the second for DC output (but most people would just rate the DC in amps). It is common for portable generator sets to have much lower output power capability in DC than in AC.
Were you thinking of
Lawnmower engine into a 12V Generator (Links)? I didn't see a Tanaka in there, but there is a Wattagan YM400, which might have a Tanaka engine... although that model appears to be strictly DC output.