Heat 1½ teaspoons olive oil in a large skillet over medium-high heat until hot. Add half of the pork; cook and stir 5 minutes or until browned and 145°F; set aside. Repeat with remaining olive oil and pork. Return browned pork to the skillet, add walnuts and reduce heat to low.
In a small bowl, combine chili powder, cumin, garlic powder and cayenne pepper; add to the skillet with water and lime juice, stirring until evenly coated. Simmer 5 minutes or until heated through.
Spoon pork-walnut mixture evenly into warmed corn tortillas, top with fresh salsa, avocado, desired garnishes and serve.