Once an engine seal starts leaking, there is no easy fix, only replacement. You can try to pick the seal out with the shifter shaft still in place although I've never had much success with that method. Another method is to screw in two  sheet metal screws into the seal to pull it from its home, but I've not had much success with that either. Always check a diagram to make sure there is enough space beyond the seal for the screw point to penetrate. The safest way in this case is to remove the drive pulley to access removal of the gearshift housing so you can put it on your workbench to drive the seal out and drive in a replacement.
One other thought I have is are you sure this is the seal that is leaking? There are two other possibilities higher up which you should eliminate as the culprit. One is the countershaft seal behind the drive pulley and the other is a rubber O Ring at the starter motor.