I had an old Yanmar
begin to steam at higher RPMs and thought it could be a head gasket as I just flushed the cooling
system and was afraid that I may have got an air bubble in the system. After performing a pressure test on the cooling
system, not finding any coolant/water in the oil or any coolant usage I looked elsewhere. The problem was a clogged exhaust elbow
which was a bit surprising because it looked to have good flow at the discharge, but boy was that thing clogged.
I would take an infrared thermometer which you can pick up for less than $40 even in the Caribbean
and take measurements of the cooling and exhaust systems of both engines. If you find a significant delta
between the two you might find your problem. As all the other posters have stated a head gasket isn't a super tough job, but rule
everything else out before you dig in.