Magnum/A-518 Swap- Speedometer?

It depends, if you use a 46 rh the speedo gear is the same as the 727 904 setup. If you use a 46 re, then you're right, the speedo is an issue. You'll either have to use an electric speedo, or a GPS speedo. I'm sure there is a way to convert that signal to a mechanical original speedo, I've just never looked.