ORIGINAL: webeflyin
I have a Saito 82a (currently on my Twist) and an OS .61FX. I think I'll put the OS on the Twist and use the Saito on the Kadet!
Either engine will have more than enough power, but I would put the .61FX on the Kadet Sr. You will have an easier time balancing the plane with the heavier 2stroke engine. I have a Saito 82 as well and it is a lightweight power house. It is precisely designed to overpower .40 ships like your Twist.