Reg,  I also have a Reig 728, and it's also greatly in need of repair.
And I agree that the quality is poor -- the case joints and all of the
glued parts (such as metal hammer shanks) are loose, and I've never put
the effort towards repairing it.  But mine does appear complete, so
perhaps I can help with information.

The missing snail cam gear is made of the same gray plastic as the
barrel, and it provides two functions: the irregular circumference
moves a hammer rail to keep the levers from contacting the barrel pins
while shifting, and the face of the cam shifts the barrel to the right.

I'd be happy to take pictures and measurements for you, if you pursue
this.  Or perhaps even sell you mine as spare parts.

Have fun!

Todd Augsburger - Roller Organs
http://www.bright.net/~allmax/index.htm