In a large skillet, heat the olive oil over medium heat. Add the shredded chicken and stir in the chili powder, cumin, garlic powder, paprika, salt, black pepper, and cayenne pepper (if using). Cook for about 5-7 minutes, stirring occasionally, until the chicken is heated through and well-coated with the spices.
While the chicken is cooking, warm the corn tortillas in a separate skillet over medium heat for about 30 seconds on each side, or until they are pliable.
To assemble the tacos, place a generous portion of the spiced chicken on each tortilla. Top with diced tomatoes, shredded lettuce, crumbled queso fresco, and chopped cilantro.
Serve the tacos with lime wedges on the side for squeezing over the top.