Depends what exactly your after, if all you want is the crane to move up and down the length of the shed, not too difficult, a pulley system comes to mind where the crane is connected to a fixed point on a cable (could use fishing line?), and a motor would push/pull the crane, but you will probably have to modify the rear of the building to accommodate the motor inside an extension type building. But if you want the boom to move across as well, I admit I have no ideas.
Here's a thought, depending on the actual construction and size of the crane, would a z motor fit inside the crane, negating the need for an external motor?