i think both of mine are stuffed due to corrosion getting into the magnet, having that expand and crack the soft alloy flywheel, I have one really good victa twin wrapped up in the shed, which hasn't been used much in its life and that one is perfect.
I think in the main concept of things, the flywheels design would last the expected life of the machine if cared for properly. However since they are more of a collectors item now many have died years ago. Cheers Joe.