Pietenpol, with a Continental engine and Grove 6" wheels. Plane doesn't get any simpler. Engine *can* be more expensive to maintain, but if you use a ~C85 or O200, it's a bit more modern and parts are easier to find. PLUS you see the reliability advantages over many conversions. No one can afford an unreliable engine.
Tires are cheap, there's no electrical system and the avionics likely consists of a single $200 handheld. Lots of them out there, if you can't make a part yourself you can find someone to help you.
All the above applies to Fly Babies as well, but I figure most folks would prefer a plane that at least *pretends* to have a second seat.
http://www.bowersflybaby.com/stories/piet.html
Ron Wanttaja