You'd need to either:
- Get a km/h->mph electronic adapter which sits behind the instrument cluster and adjusts the ~1.6x pulses to 1 pulse
- Replace the gearbox cog for the sensor and the cluster so that the pulses from the box come out as miles and the dash is in miles
In my opinion the latter is preferable, but a bit more costly and I don't know whether the odometer reading is stored on the cluster or in the ECU (a problem in both cases)
If you do choose the latter, I might be interested in buying the cog and cluster from you