On August 17, 2017, about 0927 central daylight time, a Beech A36 airplane, N48TZ, was substantially damaged during a forced landing after departing from the Louisiana Regional Airport (REG), Gonzales, Louisiana. The pilot and flight instructor were not injured. The airplane was registered to and operated by Gulf Central Aviation LLC under the provisions of Title 14 Code of Federal Regulations Part 91 as an instructional flight. Day visual meteorological conditions prevailed for the local flight, which departed without a flight plan about 0926. According to the pilot, he departed from Runway 17, following a normal engine run up and takeoff roll. After reaching about 150 ft agl, the pilot noticed the airplane was no longer climbing and lowered the airplane's nose. Based on his perceptions of a partial engine power loss, the pilot checked the throttle and mixture lever positions, both of which were full forward. After maneuvering to avoid trees, the pilot initiated a forced landing into a hay field. During the landing roll, the airplane impacted a small ridge and nosed over, which damaged the engine firewall. Examination at the recovery location revealed the ignition harness connecting the magnetos to each of the top sparkplugs had been disconnected from the sparkplugs. All twelve sparkplugs appeared new, with no combustion deposits noted. Without authorization, a mechanic who normally performed maintenance on the airplane stated he had removed and discarded the spark plugs due to several having a worn-out condition. Not realizing the airplane should not be disturbed during the investigation, the mechanic had also removed the fuel screen. Examination of the throttle and mixture control cables revealed proper attachment to their respective control arms. Borescope inspection of piston domes, cylinder wall surfaces, and intake and exhaust valves revealed normal wear patterns and combustion signatures, except for the No. 5 cylinder exhaust valve, which had a green crescent-shaped discoloration on the bottom of the valve face. After the fuel screen was returned by the mechanic, an engine test run was performed. The engine rotated and ran on the first attempt but stopped after several seconds. A fuel leak was observed adjacent to the fuel pump, with a flexible fuel hose leaking at a B-nut. The nut was tightened about ¼ turn, which seated the nut onto the fitting. The engine driven fuel pump and the adjacent oil filter adapter were stained with a blue color. A second fuel leak was observed at the throttle body/metering unit, with the fuel mixture arm and shaft bent in a manner consistent with impact damage. A second engine test run was conducted, during which power was not increased above 1,100 rpm, due to a damaged propeller. The engine was operated at varying speeds and a magneto check was accomplished, with no anomalies. The throttle was reduced to idle power and the engine ran smoothly. The oil on the oil rod was very dark in color. A logbook review revealed the engine had accumulated 163.7 hours since the last oil and oil filter change. The manufacturer recommends oil change intervals of 50 hours for an engine equipped with external filters installed.