Looking at the picture of the caliper, the brake pads look smaller than stock and the caliper most likely needs a mounting plate as well. Mounting bolt holes on the Ninja forks (center to center) is much smaller than 84mm. It's more like 4.8 cm (48 mm). I'd be a little cautious at buying a brake caliper for $25.00.
If you are wanting to improve braking performance, keep your stock calipers, change to SS lines and get some new pads.
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