Riding Mower Will Not Start

My 12 HP Craftsman riding mower (Model 917.254551) will not start although the motor will crank. I have changed the battery, coil, and throttle cable. There doesn't appear to be any spark when testing at the spark plug. There are gas vapors when the air filter is removed and the engine cranked. What should I check in order to get a spark at the plug?

By William