I have a Saito 56 on my 1/5 scale Sig Cub and it is a perfect combo. Just the right amount of power and the the little Saito will fly it for 30+ minutes on a tank of fuel. You will enjoy building that kit.
If you want a gas powered Cub build the Sig 1/4 scale and put a Zenoah G26 on it. They fly great too.

wcb