In a small bowl, stir together the chili powder, oregano, paprika, garlic powder, salt and pepper. Dredge the sirloin pieces in the seasonings, coating completely.
In a nonstick frying pan, cook the sirloin strips over medium heat until slightly pink, about 8 minutes. Add the onion and green pepper strips and saute until the vegetables are tender, about 5 minutes.
To serve, spread an equal amount of the meat and vegetable mixture on each tortilla. Top each with 1 tablespoon cheese, 1/4 of the diced tomatoes, 1/2 cup shredded lettuce and 2 tablespoons salsa. Fold both sides of each tortilla up over the filling, and then roll to close. Serve immediately.