SIG still has all the info on the Kadet II available on line if you had not seen it or can use it:
And the basic specs and setup info is here:
But they leave the prop choice up to the guy that chooses the engine and that makes a lot of sense. If you look at a glow engine prop size chart
for a .40 to .45 fuel engine it shows a 11 x 6 as an ideal size and the 10 to 12 range at practical. And you can certainly use those sizes with an electric motor too. The electric props are a little thinner and lighter because they run in an environment where there is much less inherent vibration and shock.
With an electric motor the prop choosing drill is to get a prop that will pull close to the maximum rated Amps at full throttle. The voltage, current draw, and propeller load will all balance to a point where the motor is producing enough power but not overheating. You can get a feel for what you have and the limits with a little static testing. A 10-15 second full throttle run, then stopping and a couple of finger touches to check for heat in the motor base and on the ESC and you'll have entered a whole new world of prepping a plane for flight.
And when you fly it you'll be amazed at the difference between electric and fuel flying. The power band will be much more linear across the RPM range and you can use any spot in the range at will. You'll never again worry about "losing the fire" from playing with the throttle.
So if a 11 x 6 or so is what you have now it will probably work. As always the old "less diameter and more pitch or more diameter and less pitch game" can be played until you find the one you like the best.
Here is a list of what's available in the APC Thin