The Kadet Sr. will fly just fine with a .40 two stroke, in the kit configuration they are not designed to do much else but teach you how to take off and land (which they are the best at), if decreasing the wing dihedral, adding ailerons and such, then you'd probably be better off with a larger engine, more power to carry the airplanes through rolls, loops, stall turns and light stuff like that.
I've bashed several Kadet's and actually scratchbuilt them from reworked plans, the white and yellow was powered by a Webra Silverline .40 and it was a blast to fly, the twin had 2 Webras .40's and believe it or not would not fly all that much faster than the single engine version, but the sound was great and I did a lot of crazy things with it, like taking off with full flaps with a sudden burst of full throttle, it was vertical in a nanosecond and if not careful it would loop from the standing position, the white with gray and red, I still have and is the heaviest of all at ll lbs. powered by a Saito .91, flies like a dream and rolls like a pattern plane (it actually flies like a real airplane), they will not spin or snap, but touch and goes are just great, this is one great design to make changes to, as the wing is big enough to generate enough lift for a 20 lbs airplane.