Unfortunately Northern Tool hasn't carried that gen head for a few years.
Yes, the Northern Tool gen head does fit on the Yanmar in the 831A. I have one on an 831 in my garage. Pic below.
The equivalent gen head can be purchased from Mecc Alt. I've bought a few of them from them the last couple of years to do conversions.
FYI - I produce conversions kits to do the swap which includes a custom governor controller to maintain 60 Hz, current sensors for overload protection and a transformer assembly to restore battery charger operation and all needed hardware.
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