Okay, it's an upright trike. Probably mild steel fork.
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Looks like a threaded fork, probably 1", but it's going to be either 1" (25.4mm) or 1-1/8" (28.6mm). The dimensions that also matter are the length of the tube, and where the threads start. The tube has to be longer than the your frame, obviously and the threads have to match. You can cut a tube that is too long, but you have to have threads that end inside the frame.
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The tube is usually hardened steel and if the threads have to be cut longer. that needs a special hard die that only some bike shops own, You have to order a fork with all of these dimensions in mind.
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The above fork is available from China for 80 bucks, but they also want 80 bucks shipping. It is steel though, and would allow your motor to use a disk brake. It has rim brake mounts but they're for a 29" wheel. DOn't know if it is strong enough for a 1000W motor and disk brakes. Will certainly need those torque arms.
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A threaded fork can be converted to a threadless fork, but that requires a new headset, a stem, and other hardware to hold the handlebars. I had to do this once, when I was unable to cut the threads with my own die.
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