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You are correct, replace the backplate:
The new YS91SR-3DS has a steel lining on the back plate to minimize drag to increase performance and horsepower. Back plate mounting RPM sensors will not work, because the steel lining will block the pick-up. A magnet mounted on the clutch bell or fan unit is recommended. Another option is to replace the steel lined back plate with an all aluminum back plate (YS5865, same as YS91SR), if you choose to use your back plate mounted RPM sensor.