Possibly just taking the output from the speed sensor and using a frequency to voltage converter IC like the LM2907 and then using the variable voltage from it to drive a voltage to frequency converter IC like a LM566 may allow you to make a simple analog based frequency changer. However all of the vehicles shift points will be a bit off being everything is changed by a roughly 63/100 ratio.
If you could change your vehicles shift point parameters then it may be possible to use a basic circuit like that along with basic shift point tweaks to make up for the differences.
But on the positive side the speed limiter would be at MPH instead of KPH!