Sorry to come to this evening's "debate" late - been watching the Ospreys - but there are multiple foreign athletes who could claim the title of greatest Olympian ahead of Cassius Clay:
Francina "Fanny" Elsje Blankers-Koen was a Dutch athlete, best known for winning four gold medals at the 1948 Summer Olympics in London. She accomplished this as a 30-year-old mother of two.
Emil Zátopek - at the 1952 Olympics in Helsinki, Zátopek won gold in the 5,000m, 10,000m, and the marathon, breaking Olympic records in each event. Zátopek is the only person to win these three long distance events in the same Olympic games. He had won the 10,000m at the London Olympics in 1948.
Lasse Viren - winner of four gold medals at the 1972 & 1976 Olympics. He won the 5,000m & 10,000m at both Olympiads.
To name but 3 worthy contenders....