There could be vibrations and cracks or other hidden damage making these motors useless, dangerous and worthless
Indeed, the engines could well have been damaged to the point of being useless. There are two possible courses of action following an event like this: Scrap the engines, and render them permanently unusable, or have them completely inspected in accordance with the engine manufacturer's instructions for a prop strike or sudden stoppage, and possibly scrap them.
Following a complete inspection for a prop strike, there would be no damage which would remain hidden. If a crankshaft were found to be cracked, it could be replaced - would you want to own an engine which could not be so repaired?
Though I won't extend to agreeing with what the pilot did by flying a great distance with known damaged engines, it is possible that by doing this, he saved the plane from being scrapped. If he felt that the aircraft could not be safely flown, or vibration was intolerable, he could always come back around, and land it....