Check out the BCMA 26 cc engine seen [link=http://www.bcmaengines.clearwire.net/BCMASPE.htm]here[/link] . Weight is 38 ounces. I just got mine. Haven't started her up yet, but its a great little engine.
Otherwise, check out the RCS 140 or 180 or the Roto 25 from RC Showcase. They are pretty light too, but also cost about $300 more when all is said and done.