Quickest and easiest way is to thread together a new outer with the old using a reverb hose barb tool and then push the old out with the new (from the rear). But depending on its condition and how well/smoothly the old cable is routed you may still need to drop the motor in order for the new cable to follow the old one smoothly.
If that's the case you should be able to just remove 4 T40 motor bolts and let the motor swing from the rear two rather than removing it completely. (after removing the skid plate/plastic guard)
If you remove the chainring from the spider and the shimano chain guide plate from the motor you should be able to reach the motor mounting bolts without the need for the tool to remove the spider. it's still fiddly though. I cut my chain guide's plate years back to turn it into an easily removable open 2 bolt backplate for this very reason.