Mix all of the pico de gallo ingredients in a small bowl. Adjust salt as needed.
Heat the leftover brisket in a covered pot over medium-low heat. Add enough water to keep about ½" of liquid in the bottom of the pot. Stir frequently and add more water a little at a time when necessary. Brisket will be fully reheated in about 10 minutes.
Heat 2 to 3 inches of vegetable oil in a deep 4 quart pot to 350°F.
Place a tortilla in the taco tongs and carefully fry to golden brown, about 35 seconds. Remove tortilla from the hot oil and drain on paper towels. Repeat with the remaining tortillas.
Place ¼ cup of leftover brisket into each taco shell. If the brisket is wet, just squeeze out a little of the liquid so the shell stays crispy. Top with pico de gallo, olives, cheese and green onion tops.