AFAIK, and the 400 guys that are sharper on this model than I am will certainly correct me if I'm wrong, but the 89-91 models are mechanically the same. The brakes on the twin disc models are different from the singles, but that's about it.
As for the speedo cable, I can't see why a KPH or MPH cable should be any different. The difference should be in the speedo head or possibly the drive unit on the wheel, but the cables should be the same.