Very simple process...take the old masters and transfer the sound alone to some disc. Then duplicate the Spanish masters to another disc. Then you will have two sources . One with picture and another with sound.
Now find a sync point, like gun shot, slap, crack or some bit of dialogue where you can see the sync....means words starting with "p" "b" so you can see the lips in sync and voila.. Let it run toghter...
Any decent editing software will do it. You just need to figure out the tech specks of the masters and the final output..
