If your heart is set on trying a gas engine consider this one:

[link=http://www.genesishobby.com/ywengines/modules.php?name=catalog&file=product_info&cPath=1 &products_id=1&osCsid=6e001a9308873385983f4a9920b7 0efc]YW 24cc[/link]

Supposedly weighs 37 oz. with ignition and pitts muffler (only 5 oz. more than an O.S. 120 Surpass III pumped) and turns an APC 16x8 at 8400 rpm. And it looks good, too!