You'll probably have to get a HD wiring diagram so you can see which wires are used for what purpose on their speedo. And, you'll need to pick up an I/S sender, bolts right in place where the mechanical one mounts on the front wheel. Then use the wiring from the sender and match to the HD speedo. Chances are you'll need something called a Speedo-Healer to correct for the difference in the Honda sender and the HD speedometer. You'll have to use the output from the sender, wire it into the Speedo-Healer, then the output from that to the HD speedometer. I did that on a friend's Valk, he wanted to adapt a VTX 1800C speedo and tach which I was able to do using the Healer. The link takes you the Speedo-Healer mfr. website.
Good luck, keep us posted on your progress.
http://www.healtech-electronics.com/