Numerous things float to the top. Belt/pulley/idler/tensioner/bearing issues. Plugged up carb. Weak spark/coil. Dirty plug. Water in the fuel. Cylinder full of carbon. Valves need adjustment or a valve job. Compression test. Total health and welfare check on the ol' girl. Old gas. on and on and on.
1.Does it have compression? Pull the rope. Is it hard to pull? If so move to #2.
2.Air filter usable? If so move to #3.
3.Dump and flush out the fuel tank. Install new fuel lines to the carb. Boil clean the carb. Steady flow of fuel AT the carb inlet.
4.Strong spark UNDER LOAD? Gap an old plug 1/4" wide. Does it jump that obtuse gap?