After removal of the flywheel and ignition coil, the homelite gasglow weighs in at under 3lbs. This includes the muffler. Compared with commercially available 26cc gas engines, it seems pretty competitive.

The zero has a wingspan of 70.5 inches and an area of 877 square inches (57 dm squared). It is slightly smaller in area than your recommendation. It supposedly requires a lot of nose weight so I might be able to put the flywheel back (might as well use that weight quota). I am thinking that might be a decent candidate. Apparently comes in quite heavy at 18 - 20 lbs but reputed to be a "floater with trainer like flight manners".

I am tempted to build a coroplast  giant spad stick.  Nothing quite as impressive as a plane which looks like it was entirely dug up from the junk pile but yet flies well. That one has an 80 inch wing span and apparently comes in at 20 lbs. I might scale it down by 10 inches in wingspan.<br type="_moz"/>