In a small bowl, whisk together the ketchup, marmalade, garlic, vinegar, red pepper, and the chili powder and mustard (if using). (At this point, the glaze can be covered and refrigerated for up to 2 weeks.)
Heat grill to medium. Place the chicken, skin-side up, over indirect heat, close lid, and cook for 25 minutes. Brush the glaze over the skin and continue to cook, brushing once more with the glaze, until cooked through, about 20 minutes more.