I think that last photo is showing the stator side. The inside of the bearing will be moving with the rotor. Is that a skateboard bearing (22mm outer, 8mm inner). So you’ll have a shaft going through this? I’d glue or attach magnet to end of shaft and then build a shim to hold the magnetic sensor a few mm away from magnet. Probably need some new holes in stator to attach this securely.
Some sensors come with a magnet (as5147?), others do not (ma730). You can pick up a 10x diametric magnet from ebay for £3 e.g.