Ebay often has someone parting various diesel
engines. If you contact the sellers, even if they aren't selling parts for your engine, they may have contacts. Most heat exchangers for the same hp should work. Or a larger one with a valve to restrict flow. Your engine should run at the designed temp, not too cool.
stores sell after market replacement heat exchangers.
Wire brush the rust and paint
with a good spray paint. I usually prime first with a zinc rich primer or cold galvanizing paint.
If I had the engine out, I would replace the rings, bearings, seals
and have the valves done. Probably hoses, too.