My 2155 won't start with the key. I can crank it with a screw driver across the terminals on the starter. I think it has to be either the key switch or the safety neutral switch.
I don't have a wiring diagram but I can tell where to find the neutral start switch which is probably the problem. Take out the bolts that hold the sheet metal on that the 2 gear shift levers protrude through. You'll find the NS switch under the cover. A lot of times all one needs to do is remove one or two aluminum shim washers and re-install old switch.