Combine the sugar and water in a small saucepan. Set over medium heat, add the chile piquín, stir, and bring to a simmer. Reduce heat and simmering for 5 minutes until the sugar dissolves into the water. Turn off heat and let it sit for at least an hour before using. Strain before using.
To make the Agua de Mango:
In a blender, combine the mango chunks, 1/2 cup cold water, lime juice, 1/2 ounce of the chile piquín syrup, and the tequila. Puree until completely smooth.
Wet the rim of a glass with water, or rub with half a lime, and dip into salt. Add ice cubes to the glass and pour in the agua de mango. Garnish with a slice of mango and a lime wheel.