I always, always, ALWAYS start with trying a bit of Marvel Mystery oil in the fuel and oil. It is an excellent cleaner, excellent tool to remove gunk out of carbs (yes even the teeny holes) and also helps lubricate and clean the upper cylinder & valves.
I've seen this stuff help engines that are seized function again. Basically, I see it as "top of the line transmission fluid" with super heavy cleaners.
I've used in go-karts, mowers, tractors, tillers, weed trimmers, vehicles, to remove rust, oil door hinges... The uses are vast. It also cleans fuel injection systems out and lubricates parts in fuel pumps.