ORIGINAL: Texastbird
I vote for the .91. A Dave Patrick Cub is well over 100'' wingspan and it flew great with a Saito 1.50 and had much power to spare. Now, if it needed the nose weight to balance, then stick in the bigger engine and set up the idle REAL slow so it will land, ha, ha.
On spark ignition, the 150 idles @ 1400 RPM.
According to this calculator
http://personal.osi.hu/fuzesisz/strc_eng/index.htm
The 150 turning the 18X8 @ 7850 RPM will generate 17.64# of static thrust. That should be enough to hover W/an 11.76# airplane.