Add cumin, peppercorns, and coriander or coriander seeds to a small nonstick frying pan over medium heat. Stir and cook until the spices are toasted and very fragrant (a few minutes). Let cook for 5 minutes more, then add to blender, spice grinder, coffee grinder, or small food processor. Add to a small bowl and stir in sugar and salt. Rub the spice mixture all over the outside of the pork roast and place it in a gallon-sized resealable plastic bag.
Add marinade ingredients to 4-cup measure and stir to blend well.
Pour marinade into the plastic bag, remove any excess air, and place bag with roast into a medium bowl. Chill in refrigerator overnight.
Start grill, or preheat oven to 450°F. Remove pork roast from marinade and, if using the oven, place it on a rack in a small roasting pan. Roast 25 minutes. Reduce temperature to 300°F and continue to bake for about 40 minutes more (if using a meat thermometer, it should read around 155°F when inserted in the center of pork roast). If grilling, use indirect heat and remove when meat thermometer registers about 155-158°F.
Set pork roast on a serving platter and let rest for 10 minutes. Cut roast into slices in the thickness you prefer.