We had a couple middle '90s C-series Cummins engines leaking coolant externally. One was in combine and one in a tractor. They would of had between 2000 to 4000 hours. We just had head plained as needed and new gaskets installed. Stop leak was temporary fix for both. Shouldn't be any reason to go further into engine. Of course with 3300 hours it's anybodys guess what the engine has had to go threw. Should be engine is just nicely broke in.