I doubt that there's anything wrong with the bearings. They rumble, not clack. Clacking normally has an electrical cause - something like the motor cable connector not pushed in far enough or a bad connection on the phase wwires at the controller. Tell us more about the clacking.
To get the motor core out, you rmove any nuts on the axle and tap it out with a mallet on the end of the axle. Wooden or rubber mallet is best to avoid damaging the threads, otherwise use a block of wood and hit it with something heavy. Don't forget to have something to catch it when it drops out!
One bearing is in the side-plate you've taken off. the other will be left in the far side of the hub when you get the axle out. the third one is in the middle of the motor. Dismantling that far is very tricky and not recommended, especially as it's very unlikely that there's anything wrong with it.
You're motor looks pretty clean. the only time I've seen bearings needing replacement is when the motor was full of water like this:
Hello all. Noticed of later the weather, my commute and general aggressive riding style Is beating the crap out my bike. The poor thing needs a bit of an overhaul and some love. But in regards to this I've noticed a more metallic sound coming from my hub motor. Also noticed a small wobble...
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