I mounted the engine upright because it only required a little bit of grinding on the top of the fuselage for the muffler and the carb. I mounted the engine so the centerline of the crankshaft would be in the same place as it would be if the recommend 26cc evolution were used. If you mount it inverted I think you would have to cut off the fuselage sides from the firewall forward, or mount the engine very low. I don't like the idea of mounting the engine low because I don't know what effect it may have by moving the thrust line from the original plans and I don't like the engine and muffler hanging down where it would suffer more damage from a nose low landing. I don't like the idea of cutting off the fuselage sides because I think it looks better with the fuselange intact. I have not weighed the plane yet, but I will when it is finished. I did check the RTF weight of the Zenoah G26 and the RTF weight of the recommend evolution and they were within a few ounces.