Well, I guess the new Senior Kadet ARFs are built a lot heavier than my old version of which I had to build up myself. it only needed something like a 35 or 40 engine to fly. it was more like flying a powered glider. I flew it for a long time as a electric using a Astro-Flight geared 40 motor with three 6 cell car nicad packs in series. Later after it hit some power lines and went in, I repaired it and switched over to glow fuel power with a O.S. 60 FS engine (exposed rocker arms). I wound up using a big prop to slow the motor down and only needed 1/4 to 1/2 throttle to take off and fly it around.
But at the flying field somebody has a Senior Kadet plane, installed ailerons, and mounted a 60 or larger engine on it. Sheesh.
I still have it too, but it needs to be recovered as it has gotten pretty raggidy over the years.
So you should be fine with a 45 size engine, just remember you have a throttle, it flies best when you back off the throttle and just tool around in a sedate manner.

yeah the ARF must be heavier as SIG suggests a 40 to 46 engine with the ARF and on the kit SIG suggests a 29 to 40 engine.
So there you have it right from SIG's own information.
A 60 four cycle would do nicely too.