On May 3, 2014, about 0915 Pacific daylight time, a Bell 47G, N7895B, sustained substantial damage during a precautionary landing, about 3 miles south of the El Monte Airport (EMT) El Monte, California. The helicopter was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. The private pilot and passenger sustained minor injuries. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight. The cross-country flight originated at EMT about 0910, with a destination of Compton/Woodley Airport, Compton, California. The pilot reported that during climb out, about 600-700 feet above ground level, the engine experienced a partial loss of power. The pilot stated the helicopter shook violently and the engine made a couple of popping sounds. He immediately initiated a precautionary landing at an open field. The pilot stated that during the landing, he carried a little forward speed and the right skid caught on the ground and rolled the helicopter on to its right side. Examination of the helicopter by the pilot revealed that the fuselage and main rotor blades were structurally damaged. The wreckage was recovered to a local storage facility for further examination. Examination of the recovered airframe and engine was accomplished by a certified airframe and powerplant (A&P) mechanic, under the supervision of the National Transportation Safety Board investigator-in-charge. The examination revealed that the engine remained attached through all of its mounts. The carburetor was intact and disassembled. All internal components of the carburetor were intact. The top spark plugs were removed and the engine was rotated by hand. Thumb compression was obtained on all cylinders and proper valve movement was observed. All engine control continuity with the cockpit controls was established. Both of the magnetos were intact and removed in order to functionally test them on a test bench. During the test sequence, the right magneto functioned normally; producing a spark at all leads during various rpm settings. The left magneto was tested and did not function properly. At low rpm setting, spark was not observed at the leads. At higher rpm settings, spark was observed at the leads but was intermittent and firing at random. The left magneto was disassembled and the internal rotor was observed loose from its attachment. The spark plugs were removed to be functionally checked on a spark plug tester. All the spark plugs functioned normally with the exception of the number 5 cylinder's spark plugs. The top spark plug had heavy carbon deposits in the electrode area and no spark was observed. The lower spark plug electrode area had corrosion present and the electrode was thermally damaged. Review of the helicopter's maintenance records revealed that the engine and airframe underwent their most recent annual inspection on August 28, 2013, at an engine total time of 537 hours and airframe time of 2,726 hours.