1998 wizard twin II B&S 19hp. AYP #8196A89

Try putting 12 volts battery power direct to the starter. 
 
See if the starter functions and that the engine isn't seized.
 
If it does spin over, pull the spark plugs, ground to lead(lead connected) and check for spark.
 
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Did you check the main fuse to see if it's blown? 
 
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I gather you tested it, or tried a replacement fuse?
 
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https://jackssmallengines...196a89-1998/electrical
 
Find both switches and do at least a visual inspection of them.
 
If you look on You Tube, there's several video's on how to test these switches 
with just a multimeter.   
 
If a switch has just 2 wires, either un-plug it, or jump
the wires to by-pass the switch.
 
Looking at the parts diagram, electrical system, you have a separate starter solenoid.
Do you have 12 volts constant to one side (heavy line) of the solenoid?
If so, when you turn the key to crank, is one of the smaller attached wires getting 12 volts(to activate the solenoid)?
 
If you jump (carefully) the two large wires on the solenoid, will it spin over?
 
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