Preheat oven to 220 degrees. Wash ham thoroughly and pat dry. Place ham in roaster that has a lid. Pour 1 liter of rootbeer over ham. Add half the kumquats and 1/2 cup of brown sugar. Bake covered, for 6 hours, basting often. When done, remove ham from juices and discard juices. Return ham to roaster. In saucepan, combine remaining brown sugar and 2 cups of rootbeer. Bring to a boil and reduce until thickened enough to coat ham. Remove from heat and stir in both kinds of mustard. Brush sauce over top of ham and return to 300 degree oven uncovered to let brown. Add more sauce as needed. Add remaining kumquats during final 15 to 20 minutes of cooking. Be careful not to burn top of ham. Serve remaining sauce separately.