The pilot reported that during landing on the asphalt runway, with a passenger in the front seat, the airplane had a significant tailwheel shimmy. The pilot attempted to stop the shimmy by lifting the tailwheel off the runway as the airplane continued to decelerate. As the speed decreased, the tailwheel settled back onto the runway and shimmied again, but directional control was maintained. However, the shimmy eventually became more adverse, and the pilot was unable to maintain directional control of the airplane. The airplane ground looped to the left, came to rest adjacent to the runway, and sustained substantial damage to the right wing and empennage. Postaccident examination revealed that an unapproved tailwheel assembly and tail spring were installed. The tailwheel created a negative castor angle and reduced trail, which was exacerbated by the additional weight of the rear occupant, and resulted in the tailwheel shimmy during landing.