Combine the tomatoes, chili powder, garlic, 2/3 cup water and 1/2 teaspoon salt in a small saucepan. Bring to a simmer over medium heat and cook, breaking up the tomatoes with a spoon, for 10 minutes. Add the chicken breast, nestling it in the sauce, and simmer, flipping once, until cooked through, 8 to 9 minutes per side. Transfer the chicken to a plate, shred it, and return it to the sauce. Season with salt and pepper.
Lay a tortilla on a work surface and top evenly with 1/2 cup Oaxaca cheese. Add a quarter of the shredded chicken and tomato sauce and a quarter of the beans. Fold the tortilla in half to enclose the fillings. Repeat with the remaining tortillas and ingredients.
Melt 1/2 tablespoon butter in a large nonstick or cast-iron skillet over medium heat. Cook one of the quesadillas, flipping once, until golden brown and the cheese is melted, about 3 minutes per side. Remove to a cutting board and repeat with the remaining quesadillas.
Cut into triangles and serve with the guacamole, salsa, scallions and sour cream.