RE: Vintage K&B 61 power RPM
Although the engine may get the best power with an 11-7 or a 12-6. You don't need that much power for a Kadet, and both have too much speed for a Kadet. You need a higher thrust and lower speed prop, if you think a 12x4 is too light then perhaps a 13x4, or a 13x4 cut down 1/2".