After 17 years hitchhiking across the country, panhandling for food and living in shelters and a mental hospital, a 51-year-old retarded man has found his family. ``It's a miracle,'' said Frank Caswell, who has been a patient at Eastern State Hospital since last November. He had no identification, money or additional clothes when a mission for the homeless in Norfolk sent him to Eastern. Caswell was unable to tell workers at the hospital his hometown until last month, when he said he may have lived in Dover, N.H. He told social worker Martin Phillips he thought he had a sister who was still alive. PNo one would miss me anyway,'' Caswell said Monday. But he said he was eager to return to New Hampshire and promised never to leave again. Caswell's memory loss and an unusual limp had led hospital workers to believe he had been a chronic alcoholic. Actually, the limp came from his contracting polio in 1949. The challenge of piecing together his past had been made more difficult by false leads he gave the hospital. He guessed his age to be about 40 and came up with inaccurate names of relatives. Phillips said officials hope Caswell can return to New Hampshire and be reunited with his sister in the next two months. ``Basically, for the last 17 years, he has been a mystery man,'' Phillips said.