Can someone become the king of the pirates only by finding the one piece.
- Be the strongest. ( Late Roger was the strongest pirate in his era at peak, Late Whitebeard was the closest to Roger's strength )
- Get to the end of the grand line aka Laugh tale. (Roger was the first person to conquer the grandline so he was given nickname "Pirate king".)
Also it is not a title it was just a nickname to separate him from pirates weaker than him.