On July 5, 2001, about 1100 eastern daylight time, a Stinson 108-3, N6647M, was substantially damaged during a forced landing, after takeoff from Potomac Airfield (VKX), Friendly, Maryland. The certificated private pilot sustained minor injuries, and the passenger was seriously injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal conducted under 14 CFR Part 91. The pilot reported that he purchased fuel at VKX, and intended to return to his home airport, Washington Executive Airport (W32), Clinton, Maryland, located about 2 miles to the east. The pilot added that he started the engine, taxied to runway 24, and performed a magneto check. No anomalies were observed, and the pilot initiated a takeoff roll. However, the engine began to run rough, and the pilot aborted the takeoff. He then ran the engine at different RPM, and performed a second satisfactory magneto check. During the second takeoff roll, the engine ran rough again, and the pilot aborted the takeoff. The pilot then let the engine run for 5 minutes at different RPM, and it passed a third magneto check. On the third takeoff attempt, the engine "had good power," but as the airplane climbed above the trees, the engine lost all power. The pilot performed a forced landing into trees, and the airplane settled to the ground in a wooded area. A Safety Board investigator and Federal Aviation Administration (FAA) inspector examined the engine after the wreckage was recovered. They observed that the right-side muffler baffling was mostly disintegrated, consistent with a baffling failure and subsequent blockage of the engine exhaust. The FAA inspector stated that the muffler was manufactured by Hanlon Wilson Muffler Systems, and the baffling was required to be inspected for cracks and wear during the annual inspection of the airplane. The last annual inspection of the airplane was performed on March 22, 2001, and the airplane had accumulated approximately 28 hours of operation since that inspection.