Flight Instructor reported the aircraft experienced a loss of power during a go around from an unstable approach so they took control from the student and landed the aircraft.

Narrative

Instrument pilot and acting PIC (Pilot in Command) working through the commercial pilot training program at flight school was practicing a steep spiral over approach end of Runway XX at ZZZ. The flight maneuver was relatively new to the pilot. Pilot forgot to clear the engine and instructor reminded the pilot to do so. The pilot cleared the engine several times but not as many times as would have been prescribed by the flight school standardization manual. Five rotations of the spiral were performed. The pilot was instructed to perform a power-off 180 to land when reaching traffic pattern altitude. The pilot attempted the same. When the pilot realized the landing was going to be too long to leave quite sufficient runway to accomplish the maneuver the pilot announced he was going around and added full power. Immediately the engine began to exhibit signs of failure. The airplane was at approximately 50 ft above the runway. Pilot promptly pulled throttle to idle and; leaving full flaps in; pitched down to attempt an emergency landing in the last third of the runway available and the grass beyond if necessary. There was at least three runway stripes visible and maybe 500 feet of grass before electrical wires and a line off trees off the end of runway. CFI announced 'my flight controls;' and pilot relinquished the controls immediately. CFI added full power and the engine experienced a second or two of roughness and then seemed to clear and run normally. CFI conducted a go around.Fouled spark plugs. The pilot and instructor had only flown together in this plane 3 times and had to perform a burn-off each time after experiencing roughness when checking left magneto. Pilot had not cleared the engine enough for this particular aircraft. CFI stated that he thought the pilot had cleared the engine enough at least to ensure no issues but recognized the cause of the issue regardless when it happened and responded concluding that the engine was not cleared sufficiently.Suggestions: Changing the spark plugs on Aircraft X; and clearing the engine religiously.