The pilot reported that while en route, the airplane's engine began to make an abnormal noise and was running rough. He opted to make a precautionary landing at a nearby airport. The touchdown for the wheel-landing was normal and as the tailwheel settled onto the runway surface, the pilot held aft pressure on the control stick. During the landing roll, the airplane veered to the left and, despite the pilot's efforts to regain directional control with right rudder application, the airplane ground looped. The right main landing gear collapsed and the airplane incurred substantial damage to the interconnect strut. The pilot initially reported no mechanical malfunctions or failures with the airplane that would have precluded normal operation, aside from the rough running engine. Later he submitted a statement that indicated he thought there was a possible failure of the landing gear. A postaccident examination of the landing gear revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.