Larry,
I'm sure any gauge shop can make a new face for the gauge. It's the same as the speedo in a some Ferrari's. The sending unit is also used in some Ferrari's and Alfa's of the same period. You can try Palo Alto Speedometer. They have an excellent reputation but whenever I've called them, they're backed-up and can't handle my job for months. I've had better luck with North Hollywood, which isn't meant as a glowing recommendation. The work is excellent but takes forever. I tried a shop in New Mexico but it was even worse. I think the name was MoMa Manufacturing.
If you find a sending unit from a US model, it's likely the same as the European version. The speedo face will be different, not the sending unit.
If the car owner just wants the speedo to register in MPH, the easiest way is to use an Autometer GPS sending unit. The connections to the Si gauge are easy and you can calibrate it so KPH becomes MPH.
I know this works because I have an Si speedo in my Pantera. I had North Hollywood Speedometer put the workings from an Si speedo into my OEM Pantera speedo. At first I used an Si sending unit but I switched to the Autometer GPS sender as it's accurate to 3 ft! The part # is 5289 and it's $230 at Summit.