One leg of the X would be placed directly under the engine. That puts 2 legs acting as chocks for the wheels, too. I was thinking of each leg being about 5 ft long with the lift arm being 4 ft. If that lift arm could tilt as well as pivot, then the engine could be brought close to the center post before swinging, to make it more stable as it passes the open areas of the X.
Actually treated wood would make a better base than steel pipe. It would be unlikely to get pushed into the ground. A 2x8 laid flat with a 2x6 rib down the center (upside-down 'T' in cross section) with steel brackets for attaching the braces on the center post would provide a stable base. The least stable position would be when the lift arm and load are at 45 degrees to the legs.