PedroM - The DLE 20 will be OK for the WACO, it will fly at more representative "scale" speed then a DLE 30. I was using the SAITO FG-20 and it was flying great and that engine is nowhere near as powerful as the DLE 20.
I cannot remember what mount is supplied with the engine (and I have three of them...), but I was using the Great Planes GP 60-120 engine mount on couple of my aeroplanes. I have attached photos of one of the engines I just received from a friend on this mount, and a photo of inverted installation in my Balsa USA Stingray 120.
As to the fuel tank size, while I cannot remember what I was using, I think they are more like 16oz.
Cheers,
Eran
