Any 20 year old engine, regardless of the hours which hasn't had at least two after cooler maintenance jobs, is probably in trouble. The after cooler is probably the most vulnerable part on the engine because it has sea water on one side and compressed air on the other which is burned by the engine. Lots of room for problems!!
So definitely pull the after cooler and service it. Read how Tony Athens does his Cummins after cooler on
https://www.sbmar.com/tonys-tips/ as well as a short article on the Yanmar's. With the aftercooler off, the lube oil cooler should be easy.
David