Is there a way for the helicopter rotor to get its power half by wind, half by engine?
The problem is that as an autogyro, the air is going through the rotor from below, and as a helicopter, the air is coming from above and it has to be one or the other.
In any case, gyroplanes don't have a problem of low rotor rpm as long as you keep the rotor loaded (and you always keep the rotor loaded).  If you slow down, the rotor rpm doesn't decay, the machine just descends steeper.   You can slow a gyro to zero airspeed, and it will maintain most of the rotor rpm on its own and autorotate straight down, although the vertical speed may get pretty high. If you have a landing gear that can tolerate a 500 fpm descent, and start from only 20-30 feet or so, you can land like that.  This is why some small autogyros don't even bother with a rotor rpm gauge.
So to get low rotor rpm, you'd have to unload it to take away the aerodynamic forces driving the blades around by pitching over to go zero G or close to it, and at that point an engine might be useful to keep the rpm from dropping, but you'll have probably already chopped the tail off from the blades flapping down (low G maneuvers in gyros are to be avoided at all costs - as with 2 blade helicopters, for different reasons), and you'll already be on your vertical trip to your final resting place anyway.