You could always fit a Speedo Healer. Takes about five minutes to fit and the same to calibrate it. Not only will it give you a KPH to MPH alteration it will correct speedo error too. I have found the device incredibly accurate.
We are talking accuracy that was EXACT to the sat nav all the way up to 100mph (OK 100 indicated, 99.5 actual!!!!!!!!!!!!)
It has a loom that fits the FireStorm with the removal of one electrical plug (one in the opaque rubber boot just behind the tank so easy to get to!!!!
"Only ride as fast as your guardian angel can fly" !!!
I've the same issue mate, the clock only shows KPH so a new inset is needed. I fitted a sigma bike computer which gives a steady accurate MPH speed, trouble is the little computer is slow to catch up.
The dial came the next day fitted quite well just needed to make the stop needle hole a little bigger. Took 10 min to fit and it works a treat, cost me 29.99 all in,