Start with the basics. The engine needs compression, fuel and spark in the right sequence to run.
If we assume that nothing has been changed since the last time it was run, its probably safe to assume the sequence is still correct. You can do a quick check for compression by removing the spark plugs, putting your thumb over each hole in turn, and cranking the engine. You'll feel a burst of pressure as each cylinder comes up on the compression stroke. No compression on one or more cylinders is probably stuck valves and will require removal of the valve cover.
Check for spark. Pull one of the spark plugs, leave the wire connected to it. Hold the body of the plug against the engine (not with your hands or you'll likely get zapped), crank the engine, and see if there's a spark. Should have a nice hot blue spark. If you get a weak, yellow spark, replace the points and condenser before going any further. Actually, it's not a bad idea to put in new points, condenser, rotor, and cap anyway.
No spark, remove the center lead from the distributor and hold the free end 1/8" or so from the engine block. Spark here but not at the plug indicates a bad rotor, cap or plug wire.
Still no spark, clean the points (don't use sandpaper, it'll leave grit embedded in the contacts and burn the points in short order). A nail file will work if you can't find anything else.
Still no spark, you'll need a meter or test light. With the ignition switch on, there should be voltage at the (-) terminal on the ignition coil and the engine block. If not, check the ignition switch & wiring. Assuming there's power at the (-) terminal, check for voltage at the (+) terminal. With the points open, there should be voltage between the (+) terminal and the engine block, and with the points closed, there should be none. If you crank the engine, you should see the voltage between the (+) terminal and the engine block pulse as the points open & close.
Voltage at (-) and none at (+) equals an open ignition coil or shorted wire to points.
Steady voltage at (+) with the engine cranking equals broken wire to points or dirty points.
If everything else seems OK and you still don't have spark, replace the condenser.
The (+) and (-) above assume the tractor is still positive ground. If its been converted to negative ground, reverse the (+) and (-) test points above. If it has a magneto instead of a distributor, things get a little more complicated.
Assuming you have spark, now look at fuel. First, make sure the fuel shutoff valve underneath the tank is open (you wouldn't be the first person to try to start a tractor with the fuel turned off). Second, open the drain valve on the bottom of the carburetor. You should have a steady stream of gas running out. If not, either the fuel line or the float valve in the carburetor is clogged.
Assuming you have gas at the carburetor, close the choke and crank a little bit. You should be able to smell gas coming out the exhaust if the engine doesn't fire, and the plugs should be wet with gas.
If you don't have gas getting to the cylinders, either the carburetor is sufficiently clogged that it'll need to be overhauled, or there's no vacuum in the intake.
Remove the air cleaner connection to the carburetor, put your hand over the carburetor intake, and crank for a couple strokes. There should be a strong vacuum at the carburetor. If not, you probably have a bad manifold leak. Stuck valves could also cause low vacuum.
If you have spark, gas to the carburetor, and vacuum at the carburetor intake, but still no start, you probably need the carburetor overhauled. They're pretty simple, but if you haven't done one before I'd recommend not taking it apart without a good set of instructions and a rebuild kit.
There are a number of other reasons for a no-start condition, but this should at least allow you to make sure the basics are in place.
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