Hi, skaboy.
Not too long ago I had a similarish problem with my Epi T-Birds ALUMINIUM

shafts. The spline just snapped off.
What I did, was to turn a new steel shaft. I was too lazy (and in a bit of a hurry) to set it up in my milling machine so I cut the new splines with a Dremel, by hand. Works well.
In Your case, things might be a bit more complicated. Mine was easy because the wheel was floating freely around in the case and I was able to drive the wheel and the back out with a punch. In Your case it might not be possible to move the worm in such a position that neither of them are damaged. If You can unscrew the leaf and push (gently, and rotating the main shaft as You go) the worm and the worm shaft out from the other side, you most probably can drive the main shaft with the wheel and the back out and the rest is just measuring and machining.
Also, only the top portion of the main shaft can be replaced without disassembly, but it's a bit trickier IMHO.
Those kind of machine heads may turn up in e-bay, but I wouldn't count on that.
Regards
Sam