I'm not familiar with the application, but I can give you general guidelines.

First of all, the speedometer is actually two instruments in one case...the speedometer and the odometer. The odometer calibration is changed by changing a gear. Odometer gears may be available from:

Odometer Gears - WELCOME

The speedometer is a bit more complicated. The way it works is the cable turns a bar which has a magnet on either end. The magnets ride inside a steel cup which is attached to the needle, and works against a spring. The faster the magnets turn, the more drag is exerted on the cup. This system can be calibrated by adjusting the force of the magnets or by replacing the spring, but I know of no source for these components. You can fudge it if the ratio difference isn't too great, by repositioning the needle. Use a drill to spin up the speedometer and see where it reads. Then calculate where it should read based on the speed of the drill and the axle ratio. Remove the needle, and place it at the correct reading while spinning the speedometer.

I have a writeup on my site about calibrating Smiths speedometers. This isn't directly applicable, but the concepts are identical:

Calibrating Smith's Speedometers for Borg Warner T5, Tremec and Toyota

Once upon a time, there were local speedometer shops that could do recalibration, either by rebuilding the instrument or by installing a conversion gearbox. If there's a nearby shop that can do this for you, then that's the easy thing to do...it will be worth the cost. Otherwise, Palo Alto Speedometer gets good reviews:

http://www.paspeedo.com/