I am not familiar with your particular genset but will offer the following.
If a head gasket
(O ring(s) in your case) fails a leak down the side of the crankcase is not automatic. The last time I had a head gasket
fail the leak was such that the coolant went into the crankcase only with no external sign of a leak.
Possible that sometime in the past the Faryman over-heated and the head warped. Guess I would install new O-rings and torque to specs, first. If that did not work then take the head to a machine shop and they can tell you if the head is warped and if it is possible to re-machine to fix.
On second thought, if you have the head off, take it to the machine shop first.