John Deere 850 diesel tractor flooded by water getting in from exhaust pipe

lots of work ahead, symptoms appeared to owner to be a bad starter, he purchased a new starter but still wouldn't start, I tries to turn the crank by hand.... stuck told him to open oil filler cap and oil gushed out, but when able to take off the plug it was full of water. I recommend that this motor really needs to be taken apart, but that will be a lot of time and money