One of Maine’s top-performing rural hospitals would take over one of its worst-performing ones in a Washington County merger that may leave millions of dollars in debt unpaid to retain medical care in Calais. Many of the 1,900 banks, government agencies, businesses and others to which Calais Regional Hospital owes about $25 million are unlikely to recoup much after the hospital’s proposed acquisition by its Machias counterpart. But if it goes through, the city of Calais would retain a hospital run by an organization that has operated in the black in recent years and with substantially less overhead.