Stark is not interested in food, toys or praise so much when working.
What DOES motivate him and is a new discovery this past summer is lure coursing. He has always had great prey drive (real, not toys or the like) but this summer we really got to see how great... lol. It was like it "woke him up". He can't do too much since he has an issue with his back leg, but I will run him every once in a while. It's good for his spirit.
He also enjoys tracking and has excelled at Barn Hunt.