MitchStick, helicopters have transmissions, so the problem of the blades going supersonic is not valid. You would just need to design the helicopter for a high rev engine.
The problem is reliability, and certification, as stated above.
As for the weight, the Lycoming 360 used in the beta II weighs approx 130kg. A motorcycle engine, for example the Yamaha R1 engine, producing about the same max power, but less torque, weighs about 80kg.