I love to use ginger simple syrup to sweeten chicha morada! I definitely recommend a liquid (rather than granular sweetener) like honey or agave nectar. Sweeten to your taste. We prefer ours less sweet.
If you're only using a portion of the chicha morada, you may wish to sweeten only the part you're using. We reserve the leftover portion to add to cocktails or desserts, and prefer to sweeten separately.
Calories will vary according to amount and type of sweetener, size of serving, how much pineapple is left on the peels, etc. The entire bag of maiz morado has only about 90 calories. We use about 1 tablespoon simple syrup per serving, and I used 4 servings in calculation.