Your first concern, Dave, will be to find an aftermarket speedometer unit that will marry up with the OEM drive cable. Luck to you there, amigo.
Plan B might involve checking out the options offered by TrailTech and Acewell. Both offer electronic gauges -some chrome- that contain other information provisions besides just your speed. Google 'em up.
I have a TrailTech Vapor on my R65 while my son is fitting an Acewell on his R65.