The D4G, as you probably know is a 3.6 HP. Often head gaskets come off these motors in pretty good shape and can go back on with head gasket sealer you can get at auto parts stores. If that doesn’t work, try posting a “Wanted” ad in the Classified board here. The ad is free and someone hay have a gasket – in the ad be sure to mention that Scott-Atwater made the motor for Champion. Scott made a 3.6b from before WW2, up through 1959, and the gasket might be the same. Your chances of finding a Scott part are much better than a Champion part.
Some other guys here may know of a suitable material you can make a head gasket from.
Good Luck!
Dave