I`ve got this in my MK3 -
http://www.hobbyking.com/hobbyking/s...sec_22_5g.html
Very happy with the speed and power and they feel very well build and has been trouble free after lots of use.
Might also be worth getting the XRay M18 Micro Servo Saver XR382500 , best servo saver for these little cars.
There are lots of other cheaper servos to pick from.