Air, fuel, spark is all you need. You said you changed the plug, but are you sure you are getting spark? Might be a problem in the carb or with fuel delivery so you also have to ensure that fuel is making its way to and through the carb.
Carb should be under the seat between the air box and the engine. It will have a black fuel line running to it from the fuel pump. If you pull that line off the carb and turn the key, gas should squirt out of it (not just slow drip).
Back to the basics. If you hold the coil wire close to ground do you get a spark when you crank? Pull the plug and put it in the coil wire and crank it. you should get a strong yellow/blue spark across the electrode. Is the plug wet with fuel or dry? If you're getting a strong spark, put it back in the engine and spray some carburetor cleaner down the throat of the carb and crank it. If it fires for a second or two then you know the problem is fuel.
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