Place the chicken in a large pot and cover with at least 1 gallon water. Bring to a boil, then reduce the heat and simmer until very tender, about 1 hour. Remove the chicken from the bones and cut or tear into medium-size pieces; set aside.
Strain the broth and place in a clean pot. Add the onion, leek, tomatoes, tomato juice, celery, pumpkin, bouillon, and garlic. Simmer for about 1 hour. Add the chicken, pepper, and salt to taste and simmer for 15 minutes more.
Preheat the oven to 250 degrees F. While the soup is in its final simmer, slice the tortillas into thin strips (a pizza cutter works well) and bake until crisp and golden brown. Pour the hot soup into bowls and garnish with the cilantro leaves, cheese, and tortilla strips.