I think Spinks manages to win this one and most likely by stoppage. No disrespect to Henry Cooper. He was tough as nails and if Spinks doesn’t watch himself he could get hurt by one of those lethal left hooks ( see clay vs Cooper 1 ). But Michael was just too well rounded of a fighter in my opinion.
Spinks by decision or a stoppage on cuts. It wouldn't shock me to see Cooper knock him down with a big left hook though. I know folks will disagree and they will argue that Cooney couldn't put him down. But Cooney was out of shape and IMO Cooper will have superior stamina to that version of Cooney. Spinks by decision while probably surviving a big left hook knockdown by Cooper along the way.
Spinks outboxes him and stops him, he also had a tight defence, he kept the right hand up well and he was good at moving his body subtly to deflect punches (small movement that would make the difference in a punch landing clean or completely missing) which would allow him to neutralise that left hook of Cooper’s. he punishes him at range, jabs his face in, sets him up for and walks him on to right hands, uppercuts and as he breaks Cooper down, he brings it to him and goes through the gears, puts combinations together and forces the stoppage, or knocks him out, or maybe it gets stopped on cuts before then.