The pilot reported that the cockpit canopy latch had broken at the conclusion of the previous flight. The latch was welded and reinstalled prior to the accident flight. After takeoff, the repaired canopy latch failed as the airplane climbed through 50 feet above ground level. The pilot attempted to land on the remaining runway due to the possibility of the canopy separating and striking the empennage in flight. However, he was unable to stop on the remaining runway and the airplane struck a ditch during the runway excursion. The nose landing gear subsequently collapsed and the airplane nosed over.