Time for some heavy metal! The flywheel is 100mm diameter and 25mm thick. I had an ugly lump of steel that was just right for this.
The lump was setup in a 4 jaw chuck and centered as best as possible. With light cuts to avoid the thing flying out I surfaced the outer diameter to correct size.
Then a lot of passed to cut a 5mm deep recess in the end.
After a lot of cutting I set the result into the 4×6 band saw. A jack screw served to keep the support plates tensioned so that the short stub would be clamped solidly. 45 minutes of cutting later, I have 2 pieces! The cut was only out by 0.5mm over the 100mm, a result of careful setup of the saw some time ago. With a bit of effort these can be quite accurate machines. Using my spray coolant helped produce a decent looking cut and helps preserve blade life.
Back to the lathe and centered the flywheel again. Much more cutting and we have a flywheel at last. Drill and ream the 10mm hole and then some polishing to get rid of most of the machining marks.
The result fits perfectly with no discernable wobble, as it should be.