On July 3, 2001, about 1730 eastern daylight time, a Piper PA-12, N3042M, was substantially damaged during an unmanned taxi after engine start at the Plymouth Municipal Airport, (PYM), Plymouth, Massachusetts. The certificated commercial pilot was not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the flight destined for the Lawrence Municipal Airport (LWM), Lawrence, Massachusetts. The personal flight was conducted under 14 CFR Part 91. According to the pilot, he attempted to start the airplane; however, the starter would not engage. He then exited the airplane and rotated the propeller. During the third rotation, the engine started and the un-chocked airplane began to roll forward. The airplane struck a parked, unoccupied aircraft before the pilot was able to shut down the engine. Examination of the airplane's magnetos, P-leads, and ignition switch by a Federal Aviation Administration inspector did not revealed any abnormalities. It was noted that the starter switch operated correctly; however, the starter motor gear would not thrust forward to engage the engine ring gear. During a subsequent telephone interview, the pilot stated that he did not intend to start the airplane when he rotated the propeller, and in retrospect, he felt rushed and "probably forgot" to turn the magnetos to the "off" position, prior to exiting the airplane.