The commercial pilot stated that the private pilot had just purchased the tailwheel-equipped airplane, and asked the commercial pilot to accompany him on a flight around the airport traffic pattern in order to familiarize himself with the airplane. The commercial pilot stated that the private pilot exhibited difficulty taxiing the airplane due to weakness on one side of his body from a stroke sustained several years prior. The private pilot taxied to the runway and applied full engine power to takeoff, and during the takeoff roll the airplane veered off the runway and into the grass on the right side of the runway. The commercial pilot stated that he attempted to apply control inputs to correct the airplane's path, but the airplane nosed over, and the propeller impacted the ground. Postaccident examination by a Federal Aviation Administration inspector revealed substantial damage to the engine firewall and right wing. The inspector stated that there were no mechanical malfunctions or anomalies with the airplane that would have precluded normal operation.