I have a MotorGuide on a 22' CC Xpress. See picts.
I'm not sure what the MotorGuide model number is. It was bought @ 2005. Currently, it has a simple push button that I step on to control on and off. I control left, right, forward, and backward with the hand control.

Is there a relatively easy way to add a foot pedal that will allow me to control left, right, and on off?

My best guess is what you want to do is not possible. You will probably have to get a different trolling motor.

there is not a cost effecient way to change it over best way is to get a motorguide xi5 or a minnkota terrova they have both foot pedal an remotes.