Hi Bill, did this on my 90, the electronic part unbolts completely, you need the clamp from your old tx box and the spedo cable will fit in, you might need to very carefully grind a slight taper on the end of the spedo drive cable to help it slide into the drive spindle but it will work ok, depending on the gear thats fitted the spedo might be out a wee bit also depending on what wheels & tyres you have fitted but within a few mph.
Jock