I have an OS 70 FS in mine and it needed the weight to balance but not the power. The 91 will be okay just either watch the power or set the end point on the servo to not allow full throttle. I have seen two Kadet SRs one with a saito 91 and one with a saito 100 and both flew great. The biggest problem will be vibration of the larger engine so just watch your throttle input.
fossil