Deal,
My O.S. 81a weighs 22 oz. with muffler and header, my O.S. 110a 24 oz. Is your choice based on balancing the plane, or the increased power for 3D flying? The 81a flys my 7 lb. Aeroworks 260 great with an 14x6 Xoar. I'm assuming that the Katana 50 with come in at around 6 1/2 lbs with that engine according to others that have used a Saito 82. I'll be doing IMAC abd pattern style flying. My new Extra 300 at 7lbs. 3 oz. has an O.S. 110a for giggles. With a 15x7 at 9500 and 14.5 lbs. of thrust, it should haul the freight!
Karl