Check in this order
1 firing order...ensure that the it is set up in relation to the direction of rotation and you don't have it 180 degrees off(#1 firing on the exhaust instead of compression stroke).
2 Insure it is getting fuel. visualy check to see if the acccelerator pump is squirting fuel.
3 Compression. If the engine has been siting for a long time, the exhaust valves can become sticky,causing compression to become erratic.
Other things to look for...sheared distributor(oil pump)gear roll pin,broken timing gears,stuck rings.