Scotty, the ground at the switch kills the motor by grounding out the magneto. The fuse is on the line from the Battery to the Ignition switch, so if its blowing, then there is a short somewhere downstream of the ignition. I would try disconnecting things until you find the circuit with the short. For instance, disconnect the PTO and see if the fuse blows. Another common culprit are light bulbs, they can short out to the frame and cause other odd things to happen.
A cheaper way to do the same thing (fuses cost money) is disconnect the battery and work the switch with an ohm meter connected to see which contact grounds out.
If you feel comfortable removing contacts from the ignition switch connector, or using jumper wires, you can quickly isolate the circuit with the short in it. If the contact at the "R" terminal is shorted, the alternator or alternator wire, has burned out to ground; if the contact at the "L" terminal is grounded, one of the lamps, or lamp wires, is shorted to ground; if the contact at the "M" terminal is grounded, then there is a short at the Magneto, etc. etc.
Just some thoughts.
Good luck