Whisk orange juice, peanut butter, marmalade and red pepper flakes together in a small bowl. Reserve a few tablespoons of the sauce for after grilling.
Prepare a grill for indirect cooking.
Sprinkle pork with salt and pepper; brush lightly with sauce.
Sear meat on both sides over high heat.
Cook pork over indirect heat until internal temperature reaches 150°F, brushing occasionally with sauce.
Remove pork from the grill and allow to rest 10 minutes before slicing.
Drizzle with reserved sauce and sprinkle with peanuts before serving.