RT nailed it, easiest and cheapest thing first.
Lou, "usually" a head gasket failure, with the high compression etc. of a diesel, manifests itself with overheating; the combustion gasses blow out the coolant. I think you are OK, your failure is likely just a cooler. However, water sucks as a lubricant so don't run it, change the oil to a lighter oil than "normal" and spin it over with the starter and the fuel solenoid disconnected (cooling water off so you don't fill a cylinder) then change it again before you run it. You really want to make sure water doesn't get into the bearings. It looks to me like you are due to pull out that cooler mess, lose the perforated hose clamps and replace it all with a nice new pair of coolers. You should do that regularly anyway as a cooler is much cheaper than bearings or a transmission. Give some thought to plumbing them in series, not parallel, transmission second in line, to simplify the plumbing?
Lou, if you are always in fresh water, take a look at these oil coolers, they are cheaper than marine and without any salt, no corrosion issues. I bought similar to cool my hydraulics, water in one pair, oil in the other, you will need fittings for the hoses you choose. https://www.alfalaval.ca/products/he...xchangers/doc/