At about 1200 eastern time, a Piper PA-18 lost engine power near Belmar and was damaged in the ensuing forced landing. The pilot was not injured. The airplane has just taken off from Belmar, en route to Hollywood, Fla., and was level at 1,200 feet when the pilot reported the engine smoothly lost power. The airplane touched down in a soccer field and struck a fence as the pilot was trying to avoid people. At the time the power failed, the pilot had just switched from the left fuel tank to the right inboard tank. Inspection of the right inboard tank and the fuel system revealed oily deposits the pilot described as a fuel additive. Examination of the engine revealed low compression on all cylinders, ranging from 44/80 to 5/80. Air bypass was noted on all exhaust valves and on two of the intake valves. The number two cylinder was removed and evidence consistent with detonation was found on the surface of the piston and on the valves.