The Whiz rod is a funny name for a very important part.
It is the part at the top of the rudder that guides the pull rope.
There are different length rods depending on the type of rudder housing:
If you have also lost the pin that holds the Whiz Rod together you will also need a Dowel Pin. Except for the Ski housing, the Whiz Rods have 2 Dowel Pins, but if you have smashed the top one off, you probably still have the lower one present...
Note on Fitting - To insert the dowel pin it is a force fit - work it in. It will seem like the pin is too big but in reality it is the friction between the pin and the Whiz rod that holds it in place. You can be quite firm with it - see Youtube video for how the Smart Track crew do it.