I'm not a fan of having an "always engaged" starter. It jsut seems like a waste of energy constantly turning a mechanism for nothing and it's just one more thing to break over time. I think about how fast I have to crank to get it my 6/1 started, and I can't imagine my hand turning that crank faster that about twice around per second, or 120 RPM.
That said, a 12V DC or 120V AC universal motor (like on a 120V drill) could be fitted with a small wheel with a rubber tire and mounded to a handle that's attached to the engine base frame on a hinge or similar pivot point. (Imagine a broom handle attached to the frame with a hinge.) The motor/wheel is attached to the middle of the handle and arranged so that the wheel is not in contact with anything normally, but can be moved to be in contact with the engine flywheel by moving the handle when desired. Gravity or a spring returns it when in its resting position. Since commutator motors (DC or universal) spin at much higher speeds, the wheel can be much smaller to get the desired engine RPM, giving you a greater mechanical advantage. To start the engine, you just turn on the motor, push it up to grab the flywheel and then when the engine starts, lower then handle and shut off the motor. Or engage and then turn on the motor if the motor is sized so that you don't need to initially "slip" it to keep it from stalling. Since the motor is DC (and universal motors can run on DC as well), you don't need perfect AC power and could use a string of batteries or whatever voltage is needed to get it rolling.
A similar idea would be to attach the motor to the engine base and use a belt/pulley arrangement where the belt is larger than needed and freewheels with no power transferred because of continual slip. The handle arrangement described above would have an idler pulley instead of the motor, and when you wanted to start the engine, you'd turn on the motor and move the handle to take up the slack in the belt and get the engine rolling that way. Then when it starts, you let go of the handle and let the belt freewheel again.
--Justin