Are torpedo 3 gear shift cables a standard or are there different sizes?
In my experience the normal shift cables (and housing) work fine, but there is a special end cap on the cable that fits into this hollow rod-like thing in the lower left corner of the picture. Normal cable end caps do not fit into this, because they are too thick. The original part made is out of brass and squeezed on with pliers. It's the center piece in this picture:

You can also find some hollow brass rod in a model train shop or similar and clip off a small piece. It just has to fit inside the rod.