I did it on my zero, but S is same thing. Drill new holes for the motor mount on chassis and upper deck. With the longer rear belt it last a lot longer. And the car will have more steering too. But can not use the new style motor mount as it will hit the upper deck.
With the stock belt 171mm rear belt. The smaller pulley you can use is 18T. But you need to adjust the belt to the tightest position. Which is better as you move the rear diff towards the rear give you space between the spur and the diff. So you can use bigger spur for slower gear ratio for modified motor.