Why not make it simpler. Make a mechanism like a ski-lift - two wheels with a loop of wire or string between them. If you rotate one wheel the string will move between them. Then suspend the light from a cord attached to the string. When it reaches one end it will turn and then go back again.
Something like this :
The only thing you would need is a rotating mounting thing for the light so that if it has a cord, the cord doesn't twist.