I have a fairly new tw so I have not dealt with this problem yet.
You could just buy a new oem coil assy with new wire 1195500 (1481499-001) $70.94
You could buy a used oem $9.00 or new off brand Chinese coil and wire $23.00 LINK
You could cut the coil wire really short and splice on a new section $9.00. LINK
You could try to remove the wire from the coil, measure it's outside diameter and replace it with some bulk wire from a parts store. But it may be gooped in there good.
If you can't get another wire in the coil and sealed real good, corrosion can set in and spoil a good day of riding when you least expect it. $5.00
Pic below from a 89 I think