wow, that's really old! One of the first JLG's, they made their own controllers for a while.
Good news is, that controller is made up of 4 very common micro switches, and most likely one of them failed. Check the power into and out of the micro switch that moves - when you steer the direction that no longer works. That will tell you whether your problem is a control, or further down the line.
Later JLG's used a spring loaded (return to center) toggle switch for steering. It would be quote easy to convert your machine to a toggle on the dash for steering. All you'd need is an unused hole.