The airplane had been defueled and shipped to the airport with its wings removed. The wings were reinstalled and the airplane fueled with 2-3 gallons of fuel for an engine run prior to the pilot's arrival. The pilot taxied the airplane and obtained 19 gallons of fuel prior to departing on a maintenance test flight that was to remain in the traffic pattern to perform multiple touch and go landings prior to departing for another airport. The pilot completed one takeoff and landing and no mechanical anomalies were reported. The pilot then performed a second takeoff during which the airplane experienced a momentary drop in engine power while climbing through 300 feet above ground level followed an additional momentary drop in power while the airplane was on the crosswind leg of the traffic pattern. The engine experienced another drop in engine power and then backfired before experiencing a total loss of power. The pilot then performed a forced landing on a down sloping field with tall grass. Both wing structures were damaged resulting in substantial damage to the airplane. The pilot was uninjured.