On August 21, 2019, about 1150 Pacific daylight time, a Cessna 120 airplane, N4237N, was substantially damaged when it was involved in an accident near Livermore, California. The pilot was seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, in 2018, the airplane had an issue with the “fuel leads” to the engine. After repairs, she flew the airplane back to her home airport and parked it in a hangar until its annual inspection the month of the accident. The accident flight was the first flight following the annual inspection. The airplane started normally, the runup revealed no anomalies. During the takeoff roll, the airplane took longer than expected to accelerate. The pilot reported that, after the airplane became airborne and began a shallow climb, the engine seemed like it wasn’t operating at full power despite sounding normal. The airplane stopped climbing and the pilot made a left turn for a forced landing. During the emergency descent, the left wing impacted either brush or the ground before the airplane tumbled and came to rest in an area of brush-covered terrain. A witness reported that the airplane’s engine was not sputtering, but it did not sound like it was producing full power. About 100 feet above the ground, the airplane made a sharp left turn and started to descend. The left wing impacted the ground and the airplane cartwheeled before coming to rest. The mechanic who completed the annual inspection also witnessed the accident. He reported that he saw the airplane low over the runway after takeoff and it did not appear to be climbing. Shortly thereafter, the airplane banked “steeply” to the left and impacted the ground. The mechanic reported that, as part of the annual inspection, he cleaned, gapped, and functionally tested all of the spark plugs. After the inspection, he conducted an engine run and “found a rough-running magneto.” He removed all spark plugs and observed that one had become fouled. He cleaned the plugs a second time and reinstalled them using new gaskets. He ran the engine up a second time, and it ran smoothly. During a postaccident engine examination, the spark plugs were removed from the engine. Both the upper and lower spark plugs from the No. 4 cylinder and the lower spark plug from the No. 3 cylinder exhibited oil and soot bridging the gap between the electrodes. All other spark plugs exhibited black soot build-up on the electrodes and insulator. The ignition harness exhibited damage consistent with impact and could not be functionally tested. The magnetos were removed and disassembled; no anomalies were noted with their internal components. The oil filter was removed and opened; no particulates were noted within the filter element. The propeller assembly and valve covers were removed from the engine. The engine was rotated by hand; thumb compression was established on all cylinders, and the valves moved consistently throughout. Borescope examination of the cylinders revealed no anomalies. The pilot did not complete and submit the National Transportation Safety Board form 6120.1, nor did she provide copies of the accident airplane’s most recent maintenance logbook entries.