On April 30, 1994, at 1948 hours eastern daylight time, N5907G, a Cessna 150, operated by the owner/pilot, was substantially damaged during a forced landing in Hollis, New Hampshire. The forced landing followed a loss of engine power. The student pilot, the sole occupant, was seriously injured. Visual meteorological conditions prevailed and a flight plan was not filed. The flight was operated under 14 CFR Part 91. The departure point was Nashua, New Hampshire. The pilot after practicing some manuevers and doing a practiced landing at Hillsboro, New Hampshire was returning to Nashua to land at Boire Field Airport when the engine lost power. He tried to do a forced landing in a field, but the airplane struck trees and descended to the ground. According to the pilot, he was doing a straight-in approach to runway 14. The airplane was in a descent and the engine power was set at 2100 rpm. About three miles from the airport, the power loss occurred. The pilot stated he headed toward a field bordered by trees to do a forced landing. The pilot realized that he would not reach the field, so he headed toward and between two trees. The wings struck the trees and the airplane descended to the ground. FAA examination of the airplane revealed "foreign matter contamination" in the fuel system at tanks, belly fuel drain fitting, fuel strainer\bowl, and carburetor float chamber.