Much like a potential Tunney vs Sharkey match-up, I see it being close early, but Tunney's greater determination to win would allow him to take over late. Tunney would be made to miss, and get hit with some counters, but he would also land his own telling punches due to his greater handspeed. In the end it would come down to the mentality of the fighters, Young was a crafty survival artist while Tunney was a winner who barely knew losing, and he wouldn't lose here.
Maybe i'm just being mean, but I don't see many fighters of Young's level beating Tunney on points period. He was a very complete boxer who beat a wide variety of styles at different weights, was a superb ring general with great lateral movement, defensive and offensive skill, combinations and body punching. I just don't think Jimmy Young can be legitimately compared to a guy with Tunney's demonstrated skills and record.