You need to determine if the problem is not getting fuel to the cylinder or not getting fire to the spark plug.
An easy way to do this is to engage the parking brake, shift to neutral, engage the choke, and crank the engine over 4-5 times. Remove the spark plug and inspect it. If the plug is wet with gas, the problem is no fire. If the bottom of the spark plug is dry, the problem is no fuel.
Your tractor has 2 safety switches that can short out the ignition module and kill the spark to the spark plug. If the spark plug was wet, remove the 2 black wires from both switches and crank the engine. If the engine starts one or both switches is bad.
The clutch/brake switch is mounted on the frame near the clutch/brake rod. The switch is made or broken when you push the clutch/brake pedal down.
The attachment clutch switch is mounted on the bracket for the clutch engagement lever. This switch is made or broken when you engage or disengage the deck.
Leave the white wires on the switches when testing. Remove only the black wires from the switches.
Fuel additives are not recommended for air cooled engines. Products such as Sea Foam will cause deposits on the valves and damage the carburetor. I suggest draining the gas and putting fresh gas with stabilizer in it in the tank. These engines run much hotter than water cooled engines and additives don't work well in them.