AFAIK, the 10/22Mag hasn't been produced for a good while, and they are pretty rare on the used market (hence the aforementioned $750 price tag). Also, I am almost positive that 10/22 stocks will not fit, as the magnum receiver will be longer.
I looked into this really hard a while back, but as has been mentioned discovered few have been able to make one work worth a damn. Kel-tec is supposed to be putting out a .22mag rifle, but it could be a year or two before you could actually get one in your hands. They tend to design a bunch of innovative guns without any practical means to produce them, so we end up having to wait forever lol.