I have spark and gas

Good question! You would think that you'd only need spark and gas.

In order for a piston to move under pressure there needs to be pressure first. So the possibility that there is not enough pressure in the cylinder for the Piston to rebound back and forth through a combustion cycle could be the issue.

Another issue that I could think of would be not enough air in the combustion chamber to allow for combustion to occur. If this is a carbureted engine, I would make sure that the choke and throttle are not completely closed or completely open for that matter.

For me those two reasons no pressure (bad head gasket) and no air (bad carbonation) would be my recommendation to take a look at and rule out before looking at other issues.