I have done at least three engines, where I added a flywheel. I bought some plastic rod about 1" in diameter. I sliced it and drilled out the center. After carefully adding it to the motor shaft, I simply ran the motor and filed the plastic to get it in balance. I don't have a lathe. Support the opposite end of the shaft while your driving the flywheel on.
After it filed smooth on the surface and both sides, I add the tach tape.
