To make marinade: combine yogurt, onion, lemon juice, curry powder, chili powder, salt, paprika, garlic and tomato paste. Blend well. Pierce roast several times with a dinner fork. Place meat in a heavy-duty reseable plastic bag and cover with marinade. Seal bag, pressing out all the air. Refrigerate in a bowl overnight or up to 24 hours, turning roast several times. When ready to grill, remove meat from marinade. Using a barbecue, roast meat over medium coals on a rotisserie for approximately 25 minutes per lb. for rare, and 30 minutes per lb. for medium.