If you are interested in doing a single servo conversion, here are the parts you will need.
Proline Servo Arm
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXJVG1&P=7
Heavy Duty Saver Spring
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHGE0&P=M
Servo (This servo is what I like to use. There are cheaper options that put out less torque. It's a matter of what's in your budget.)
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXTHR3&P=ML
Remove the old steering setup and install the new components.
Be sure to set the steering endpoints and trims back to zero on your transmitter if applicable and plug in the new steering servo so it gets centered before installing the servo arm.
If you want to do this conversion it is a really easy process that is self explanatory once you get started. The exploded view in your manual is helpful. I can walk you through it if you decide to go this route.