Just pontificating here with a CL in my hand:
So assume cooler leaked, engine was warm and some water/glycol came up the pushrods and onto the rockers before it was last shut down. IMNSHO, the water could have promoted all the corrosion, primarily because under the valve covers was exposed to air for however long it was parked (how long again?). The bottom end and or bores might not be that bad as they got an oil or diesel bath and maybe didn’t corrode. I mean half of the rockers look pretty good, I’m guessing from a coating of oil that never got washed off.
It sounded like it ran pretty fair, I’m just saying it would be interesting to mark all the pushrods/rockers/hardware showing any sign of corrosion, pull them off, Brad blast or media tumble them to clean them up, put it back together run it. You could cut open some filters and/or do a Blackstone analysis. Fun experiment that won’t really cost you anything but time.
You could do a compression test too, but what’s the fun in that?