The pilot of the tailwheel-equipped airplane had just completed an uneventful local flight, taking off and landing in a ploughed field adjacent to the home of his friend. A short time later he prepared for the return flight to his home base, and prior to departure he moved his seat forward to accommodate the passenger behind. During the takeoff roll he inadvertently moved the mixture control with his left knee, and the engine began to lose power. He aborted the takeoff, and as he applied brake pressure, the airplane nosed over, which resulted in substantial damage to the wings and fuselage. The airplane had recently been rebuilt, with the mixture control positioned in the lower left corner of the instrument panel, just above the pilot's left knee. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.