The 857 uses a 3 pole 70A ignition switch. None of the poles are grounded. One pole is Battery, one is Ignition, the other is Start.
Momentarily place a heavy gauge jumper between the Battery terminal and the Start terminal. If the starter turns over normally, it is probably the switch. If not, it either bad wires, bad grounding of the starter, or worse, a bad starter. Check ground from battery to the frame, and frame to starter.
If you need a switch, they are no longer available from Toro, but Cole Hersees has one. Part number is 95521-B. They run about $30.00 plus shipping.