Process of elimination. Air, fuel, spark.
4-strokes are the devil's work, but good news on checking the timing belt!
Check the fuel next. The primer bulb diaphragm can fail on any fuel supply. Quicksilver is as good as any. Assuming this is electric start, then check the wiring to the choke solenoid, and ensure it's actually opening the butterfly valve. Also check the dead man's lanyard.