"THE HEAT EXCHANGER".....sounds like a good name for a country song.
The heat exchanger is back (first line?) from the shop and reinstalled and working fine. I ran the engine a couple hours today while tied to the mooring and the coolant level seems to be holding ok. We will take the boat out tomorrow for a run at high RPM and see how it does. I'm slightly hopeful.
The shop, Radiadores Alaska, pressure tested it, tweaked it somehow (I don't speak Spanish and the mechanic doesn't speak English), retested it and says it is all ok. The charge: $55. I paid more for the taxi ride.
Maybe I can forget about buying a new engine and get on with crossing oceans.