That's a bummer.
Plan B: I wouldn't give up without investigating other Nissan transmissions that might offer a direct swap of the final drive or the 5th gearset. I bet there's something out there that'll fit.
It won't be a 1000 RPM reduction at highway speeds, but on the other hand, junkyard/off-the-shelf parts would be a fraction the cost of custom machining.
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View my fuel log 2015 Micra S manual: 5.0 L/100 km ... 56.5 mpg (Imp) ... 20.0 km/L ... 47.0 mpg (US) ...