Sears Riding Mower Won't Start?

My Sears 20hp riding mower won't crank. I replaced the starter and solenoid. Every time I try to start the 20 amp fuse keeps shorting out. What next? This all started after I replaced a leaking gas tank. It is very frustrating. There is no crank when I turn the key. When I cross starter the solenoid with a screw driver starter spins, but won't engage the flywheel. I have spent over $200.00 with no results.

By ET