It's a while since I did one of those, but isn't it a balancing weight, for dynamic balance as with tire balancing on a vehicle? If so, look for the footprint of the original mounting and glue it back in the same place. If that's not possible, the motor will run without it, maybe with a little more vibration and W&F, assuming the electronic part is functional.
I can't trace any glue residue anywhere on the black wheel.That may well be the case that it’s a balance weight (looking at where the sensors are they would be in the middle of the magnet). If so there will be a glue bed the piece should mate with - a little high strength epoxy would hold it.
As for the motor not working that’s another story. You’ll need to check that the motor is getting the correct supply voltage, p9 of the service manual, also that the motor shaft moves freely and that the bearings haven’t seized.
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