If we need a "first parameter" to start the design, I vote for the total amount of fuel carried. Let's say one gallon. For a 12 hour flight, our consumption will be .18 oz/min. I think this sort of calculation narrows the engine selection quite a bit. I have a Saito 72, and it probably burns close to this at idle. I would opt for a smaller engine. The OS four strokes seem more mild than the Saitos. It seems to me that they get better fuel economy too. Remember, BSFC and reliability will be more important than power/weight for this design.

With 6 pounds of fuel, our total airplane weight should be around 10-12 pounds. Can we get a 12 pound airplane off the ground and only burn .18 oz/min at cruise? It almost seems possible. Engine selection will be critical.