The pilot of the airplane reported that, during the landing roll the airplane veered left unexpectedly. The pilot applied corrective action with rudder input, but the airplane continued to veer left, exited the left side of the runway, impacted a ditch, and nosed over. The airplane sustained substantial damage to the right-wing lift strut, vertical stabilizer and rudder. Postaccident examination of the airplane revealed that, the bolt, spacer, nut and cotter pin connecting the upper torque link and the lower torque link was missing and not observed with the wreckage. It further revealed that the upper and lower torque links were not damaged. Damage to the nose gear and the right main gear was consistent with impact damage when the airplane exited the runway and impacted a ditch. A review of the airplane maintenance records did not reveal any recent maintenance conducted on the nose landing gear torque links.