I have a Nossen 1/4 scale Aeronca Champ , it came with an OS 140, don't care, I want to fly gas. I put a CRRC 40 kit engine in it. That engine has respectable power to weight ratio and is very cheap. Initially it did not like the idle so I used an RCEXL ignition instead of what CRRC gave me. It runs a little smoother that way. Anyway, your Decathalon is about the same size. With the 40cc and an 18 x 10 prop I can go infinite vertical or just poke along at 1/3 throtle. It is a nice set up. In initially flew that airplane with a weedwacker featherlite 26cc. It wallowed and did not climb well. I hated it. You can always throttel back.