Place the sugar in a small plate. Rub the rim of a lowball whisky glass with the lemon wedge. Dip the rim of the glass into the sugar.
In a shaker, vigorously mix the ice cubes with the rum, lemon juice, marmalade and allspice. Fill the glass with more ice cubes. Strain the rum mixture into the glass. Garnish with the kumquat slices threaded onto toothpicks and the thyme sprig.