Marinate pork: In a bowl, mix achiote paste, pineapple juice, garlic, chili powder, cumin, and oregano. Add pork and marinate for at least 2 hours or overnight.
Cook pork: Heat a grill or large skillet over medium-high heat. Cook pork slices until charred and cooked through, about 4-5 minutes per side.
Grill pineapple: Grill pineapple chunks until caramelized, about 2-3 minutes per side.
Warm tortillas: Warm corn tortillas on the grill or in a dry skillet until soft and pliable.
Assemble tacos: Layer sliced pork on tortillas, top with grilled pineapple, diced onion, and cilantro.
Serve: Serve immediately with lime wedges for squeezing.