If you are stuck on these to engines, I'd say the 56 as it has more power. I am not sure if Saito still produces the 50. You'l probabaly need more weight up front. Move everything you can forward. The 56 will fly this airframe great. Some may sugguest a 72 which will give you the option of floats later down the road.
Use the 2 stroke fuel you have been using. 18-20% oil synthetic or synthetic/caster blend.