On August 30, 2019, about 0837 mountain daylight time, an amateur built Hall Kitfox 532 airplane, N87TJ, was substantially damaged during the landing roll at the Nampa Municipal Airport (MAN), Nampa, Idaho. The private pilot was not injured, and the passenger sustained minor injuries. The airplane was registered to an individual and operated by the pilot as a Title 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from the Caldwell Industrial Airport (EUL) about 0800. The pilot reported that, during takeoff from EUL, the airplane had a hard shimmy right before liftoff. After an uneventful flight, the pilot landed the airplane smoothly onto the main landing gear. When the tailwheel touched down, the airplane immediately became uncontrollable and unresponsive to any pilot corrections. The airplane was rolling towards the right side of the runway, so the pilot braked in an attempt to slow the airplane. The propeller contacted the runway surface before the airplane exited the runway, nosed over, and came to rest inverted. The pilot further reported that, about 6-8 weeks before the accident, the owner put a larger tailwheel on the airplane because he was prepping the airplane for backcountry airports. The original tailwheel was about 3-4 inches in diameter and about 1 inch wide. The new tailwheel was about 6 inches in diameter and much sturdier. An onscene examination of the airplane was conducted by a Federal Aviation Administration (FAA) inspector. The inspector reported that the airplane was equipped with a 1 ¼ inch tailwheel leaf spring. The new tailwheel, which was designed for a Maule airplane, was designed for a 1 ½ inch tailwheel leaf spring. Therefore, a shim would have needed to be installed in between the tailwheel and tailwheel leaf spring to ensure a proper installation. In addition, a longer bolt would have needed to be installed to ensure the bolt would properly secure within the locking nut. The examination revealed that the bolt was still present in the leaf spring, but the locking nut was not present and was not located along the runway. The washer was found underneath the tail, and there were no indications that a shim was installed. The bolt was removed from the leaf spring and measured; it was not long enough to reach the locking portion of the locking nut. The mechanic reported to the FAA inspector that during the airplane's last condition inspection, which was conducted on November 24, 2018, the original tailwheel was installed on the airplane. He never inspected the new, larger, tailwheel after it was installed, nor was he required to until the next condition inspection.