strip carb, clean every lil hole, orifice and crevice. blow out with compressed air.
flush the tank with a swig of clean petrol.
run a fuel filter.
pull spark plug, check for spark, foreign bodies between electrodes, etc. maybe replace if all else has failed.
if it has spark, before replacing carb, put a few drops of petrol down the intake. it should fire up and run for a second or two if all is good.
otherwise, check valve clearances.
if it has no spark, try a new plug, and failing that, get a new coil.