Thanks Ron,
I ground a 5mm square on some 8mm rod stock, chucked it in a normal 500w electric drill, and stuck it in where the weedwacker shaft normally goes in the original nut. Worked well but unfortunately vibration broke the sidewalls of the nut socket before the motor started so maybe I'll try your idea, or maybe just grind enough of the post flange on the new flywheel to be able to mount a spinner/backplate.