I've had some thoughts about this, it's something I've been thinking about for a year or two, but sadly I have only got as far as making the box and a steel "bell target" face so far. I was going to have a plate behind the hole in the centre of the target that tips back when hit and activates a reed switch, which activates a small geared motor that resets the falling plate. The shaft from the motor will have a round cam on it with a microswitch that keeps the motor on for a while to enable it to return to the starting position after each hit and lights a flashing LED to indicate a hit.

Maybe I've missed something but that seems like the simplest way to me.