OK, let’s get really desperate : Pull the plug, squirt a tad of starting fluid into the cylinder, replace plug and crank. If it snorts or fires, the problem is in the fuel delivery. If still nada, your timing must be off, go back and check Joesnuffys excellent suggestions. I think his crank seal idea is probably right on, it’s a typical result of running your ‘air-cooled’ engine without a functioning impeller…