I think the question was the largest, not the heaviest. So what we want is the largest single engined piston aircraft weighing up to 5700 kgs. Hmmmm, could be the Gyppsland, maybe the An-2. Good question, and one which would probably be answered fairly rapidly on the Aviation History & Nostalgia forum.
In fact I've just been and asked it there, so we wait with baited breath......