On recovering an old leather covered plastic saddle.
Sergio is correct, a leather with a soft hand is needed. A concave V dart is also needed at the nose to take up the bulk that accumulates there, best if sewn closed. As far as glue, 3M makes some great stuff, but not cost effective if you are not a commercial user, Sand the back of the saddle first at the glue perimeter, A good contact cement should work, especially if it is not VOC compliant. As tugging is required, I have always mounted the seat to a post and fixed that to a bench vise for control, mount the saddle perpendicular to the post, not as it would be mounted for riding.
If the saddle was padded, be very careful upon original cover removal, if it had "gel" zones, be prepared for Goo residue.
Strong fingers help.
John Jorgensen
Torrance Ca USA