On March 2, 2007, about 1835 eastern standard time, a Cessna 150G, N682RA, registered to a private individual, operated by Dean International, Inc., collided with a powerline then the ground during a forced landing following loss of engine power, near Miami, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 Code of Federal Regulations Part 91 instructional, local flight from Kendall-Tamiami Executive Airport (KTMB), Miami, Florida. The airplane was substantially damaged and the certified flight instructor (CFI) and student pilot sustained minor injuries. The flight originated about 1730, from Kendall-Tamiami Executive Airport. The CFI stated that after departure while performing maneuvers, the engine experienced "...some short lived vibrations" which he attributed to carbon fouling of the spark plugs. The flight proceeded to the Homestead General Airport where an uneventful precautionary landing was performed. While on the ground, he performed an engine run-up and noticed the left magneto was operating "rough" and the right magneto was "fine", but everything else was "...fine...." He then operated the engine to a high rpm with the fuel/air ratio leaned. He then checked the left magneto drop and reported it was within "tolerances of 150 rpm." The owner of the flight school later reported the CFI did not contact them to obtain assistance following the precautionary landing. The CFI further stated that at that time in his opinion, the aircraft was operating normally and he elected to depart to return to KTMB. During climbout with him at the controls, the engine was operating at full rpm and during a shallow climb out when the flight was approximately 6-7 miles from the destination airport, the engine vibrations began and the rpm fluctuated 200 rpm, decreased to 2,000 rpm, then further decreased and remained at 300-400 rpm. He maneuvered the airplane towards a grass airstrip that he knew about, but because of the low light and visibility restriction, he flew past the airstrip. Due to the low altitude, he elected to land in a field perpendicular to the airstrip. While approaching the field, the airplane collided with powerlines then the ground, and nosed over. Examination of the engine following recovery was performed with FAA oversight. The examination revealed impact damage precluded operational testing of the engine. The fuel system was checked with no reported discrepancies. Examination of all spark plugs revealed all tested acceptable, with slight oil fouling caused by being inverted. Both magnetos were removed from the engine, and bench tested also with FAA oversight. The left magneto was noted to operate intermittently while on the test bench, and the right magneto was found to operate normally on the test bench. Disassembly of the left magneto revealed the point cam was worn, and the points exhibited erosion. Additionally, the coil tested out of limits. The condenser tested within specification.