You can buy a complete timer unit, remove the timer from the box, and install it into your existing box:
Intermatic T104R
Intermatic sells the mechanisms alone, but you can buy the complete unit just as cheaply. Swapping mechanisms can save you some rewiring work. However, if your BOX isn't in good shape, you wouldn't want to do so.

Again, if you by the entire box, you can swap EITHER the box OR the just the mechanism.

However, as Charlie notes, the mechanism itself isn't really repairable.