It's a bit difficult as there is several variants of drive and cables. I am familiar with the chain type and there is just a matter of hooking the link in a hole of the drive. I suggest you take contact with
http://www.speedoshop.net/
if you're in USA or
http://www.wsm.de/produkte/26/Tachometer/24/Tachometer_mit_Fliehkraft_Messwerk.html
in Germany, but there is several other specialists in various countries,
http://indiangauge.com/INDIAN%20PARTS%20FOR%20SALE.html