I fitted mine with a bluebird Bms-621mg servo. The servo is the same size as any standard servo. ( futaba, hitec, etc..)
Here's what you do: in the kit there are several plastic spacers for the servos. Take the 2 thickest ones with 2 medium ones. Install on each side -- one thick + one medium and screw your steering servo in the mounts. Now install the ball end on the opposite side that the instructions indicate ( ie: ball end facing servo or rear of car instead of front of car).
It's a perfect fit and works very well.