Marinate the chicken:
In a medium bowl, mix the lime juice, chile powder, garlic, salt, oregano, and pepper; whisk in the oil. Add the chicken, cover, and marinate in the refrigerator for 1 hour but no longer than 1-1/2 hours.
Make the filling:
Prepare a medium-hot fire on a gas or charcoal grill or set a large, heavy skillet over medium-high heat for 1-1/2 minutes. Remove the chicken from the marinade, shaking off any excess. Grill the chicken (or sear it in the skillet with 1 Tbs. oil), flipping after 4 min., until it’s just firm to the touch and cooked through, about 9 min. Let the chicken cool and then chop it into very small pieces.
Heat a skillet over medium heat, add 1 Tbs. oil and the bacon, and cook, stirring frequently, until the bacon just begins to brown, about 6 min. Turn the heat to medium high, add the chiles and onion, and cook, stirring frequently, until they begin to soften, about 4 min. Add the chopped chicken, cilantro, and lime juice and stir constantly until the chicken is hot. Season with salt to taste. Sprinkle the cheese (if using) over the top, take the pan off the heat, and let the cheese melt.
Set the skillet with the filling on a trivet on the table next to the hot tortillas, guacamole, pico de gallo, and tomatillo salsa so each person can assemble his or her own tacos.