The stock pinion is good for the 540 motor that comes with the kit, I would suggest getting some 48dp pinions and spur gears.
You will be using a much smaller pinion gear for a modified motor. For example, my gear ratio is a 72t spur gear and a 24t pinion, but I use a 27t motor which is a lot slower.
The final drive ratio (spur gear,divided by the pinion gear x the internal gearbox ratio of 2.25) is a useful formula to learn how to gear motors to, as its commonly used for racing. But seeing as I dont race modified im not much help to you there
Hope the rest helps though