Corrosion on the top of the affected piston, if it's set long since it happened. Also if it leaked into the crankcase the oil should have a lighter color, foamy look to it. Might even see some water particles on top of the oil if you look closely.
Diesel used to clean up an engine wont hurt anything internally. If you really don't plan to break it down and refresh this engine, then the diesel can be used and if you get it running just change the oil again after a few hundred miles.
I personally wouldn't buy a used engine not running, and not go through it to freshen it up. That's a small amount of time and money to ensure it's worth putting in your car.