How can I replicate the flavor of Mountain Dew?
OK, this is dangerously close to a recipe request, but far enough away that I feel OK to answer.

Use the rule of two.


2 cups of sugar  
2 two oranges  
2 limes  
2 lemons  


Juice and zest the whole fruit.

Strain and and fill to 1 gallon with water. Filter if you want it to be clear.

Carbonate using whatever technique you normally use whether that's a Cornelius keg or soda stream.