If you changed the coil and spark plug. the only thing left is the valves or head gasket. put a compression gauge on that cylinder and see if it holds compression. if not you may have a valve stuck or not seating properly, a broken push rod, or a blown head gasket. all are easy to fix and relatively cheap. just fixed one the other day. broken push rod. runs like a new one now. i think the parts were like 15.00 dollars.