Now that’s a patriotic streak! Caitlyn Jenner, 66, will appear on a summer cover of Sports Illustrated wearing “nothing but an American flag and her Olympic medal,” a source tells Hot Stuff. The I Am Cait star (formerly known as Bruce) set a world decathlon record at the 1976 Montreal Summer Games. To celebrate the 40th anniversary of her win, she’ll pose with her gold medal for the first time post-transition, adds the source: “She’s excited about that.” (Her ex-wife Kris, 60, appeared topless with a gold medal during a 2007 photo shoot on Keeping Up With the Kardashians.) Until now, Caitlyn says, she had stashed her “most prized possession” at the bottom of her makeup drawer. The father of Kylie, 18, Kendall, 20, Brody, 32, Brandon, 34, Casey, 35,…