If your fat bike has 175mm wide axle in the rear, you want the same type of motor. Here's a typical kit, but the connector is not waterproof.
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I've been looking for a water system myself, and they're hard to find as a kit. Your suggestion is a good find, but you will have to buy the cable harness, as well as the brakes, throttle, PAS sensor. Won't be cheap.
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That would mean that you would not use any of the electrical bits that came in the kit.
Maybe you look for a bare motor like this, and ask them how much it will cost with a 26x4 rim and spokes.
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You could always take the old motor out of your broken ebike wheel, and have the new motor spoked by a shop. If they complain abiut how hard it is, jeez, I'm no expert and just did a fat bike wheel last weekend. Cost me $36 for spokes and an evening. Got a 1mm wobble stiil in it, but it's good enough for me.
Final caution. You'll have all waterproof connections, but the motor itself will still take in water. My own fatbike, never been ridden on a beach, but has been ridden a few times in winter where we put salt on the highways to melt ice. Well, that salt got inside my motor and corroded the speed sensor.
