The Ringer (2005)

Pressured by a greedy uncle and a pile of debt, lovable loser Steve Barker resorts to an unthinkable, contemptible, just-crazy-enough-to-work scheme. He pretends to be mentally challenged to rig the upcoming Special Olympics and bring home the gold. But when Steve's fellow competitors get wise to the con, they inspire him to rise to the greatest challenge of all: becoming a better person.