Sounds like #4 exhaust hanging open. That would explain the bang, when it runs out of gas. Hopefully the push rod just dropped out of the rocker arm when the valve stuck, and did not bend the push rod. Seems one of the other three should at least give a little poof, if getting fuel, and valves half way decent.
Had this tractor been sitting a while..?? From the looks of the picture in your avitar, looks like it was just painted/restored..?? Was it that way when you bought it? Just wondering if maybe the manifold to gasket was about shot, then a clean up with a power washer, or steam jenny took away what was sealing it.