On October 4, 2018, about 1830 central daylight time, a Cessna 205A, N8495Z, was substantially damaged when it was involved in an accident near Wagoner, Oklahoma. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 flight test. According to the pilot, he was performing a post maintenance flight test. After a routine engine run-up with no abnormal indications, he departed Hefner-Easley Airport (H68), Wagoner, Oklahoma. The pilot climbed to about 2,500 ft mean sea level and made about three circuits around the airport area; he stated that the airplane climbed well and that all flight parameters were normal. The pilot announced his intentions of leaving the airport area and turned south to continue flight maneuvers and performance checks. The pilot started a left turn when a “very minor engine mis[fire]" occurred. He scanned the instrument panel, and all engine parameters appeared normal. As the pilot continued the turn, the "engine mis[fire]" began to increase and the engine began to shake. The airplane was unable to maintain altitude and was descending about 150-300 ft per minute. The pilot made multiple attempts to regain engine power, but the engine then lost total power. The pilot slowed the airplane to the minimum controllable airspeed and impacted trees and brush. During the landing sequence, the left and right wings buckled. A postaccident engine run was performed and revealed that, after running for 13 minutes, the engine lost power and the No. 6 cylinder began to significantly cool. Troubleshooting of the magnetos found that the right magneto was firing erratically. The right magneto was replaced with a known-good magneto and the engine was subsequently successfully test run at full power. The accident magnetos were removed for testing, and both magnetos failed the manufacturer’s acceptance test procedure due to excessive point gaps and secondary coil resistance. Additionally, both magnetos did not produce output until above 3,000 rpm. A review of the engine logbooks revealed the magneto points and condensers were replaced in both magnetos on September 20, 2000, at a tachometer time of 786.5 hours. There were no logbook entries detailing a magneto 500-hour inspection. Since the repair of the magnetos, the engine had accumulated about 233.3 hours prior to the accident flight.