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The linked gear looks like it will fit with the large part next to the motor, but it might be necessary to cut a pocket out of the underside of the spring arm to get the hob groove fully aligned with the filament path. The source is attached to this Thing if you want to do that.

The MK8 should fit fine once you put filament in between the idler and gear -- the extra thickness of the filament stands the arm away from the set screw. Unless you are using an unusually long set screw... you can always try a smaller one. McMaster has a good selection.

I agree, 22mm spring is not long enough, unless you're using a very large diameter drive gear to push the idler arm more.