First check if you actually have spark at all. Next if you have spark check that the timing isn't out by 180 degrees. If all seems fine on the ignition front,it's time to check whether the carb gets fuel into the float bowls, if it won't, you're looking at fuel pump or lines. If you get fuel into the carb it's time to see if it will fire up on start pilot spray. If that doesn't work it means that you have (decently timed) spark, air and fuel going into the engine and the only remaining thing to check is compression which should be at least 100 psi for every cylinder with as little difference between them as possible. If any of them is below 100, pour a teaspoon of engine oil in that cylinder and retest, if compression goes up, you're looking at putting in new piston rings, if it doesn't you've got valve problems.
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