Hard to say. I think Spinks could get clipped by many top ten heavies before winning a title shot. Could he out box others to gain a title shot? Maybe. But his margin of error vs the top guys would be thin. Spinks was terrible vs Tyson, and never fought again. I think other 80's heavies like Witherpsoon, Page, Berbick, Tubbs, Thomas, and others defeat Spinks.
I think Spink's loss to Tyson needs to be considered as a laydown job. Sure, he would have lost early anyway, but he clearly did not show up to take a title away from anyone that evening. That being said, I think he might have a chance at outboxing men like maskaev, Peter, Ibragimov, Povetkin and maybe even Chagaev. These men weren't much Bigger than a prime Spinks at heavyweight. Michael was 6'3", and weighed anywhere from 200-210 Lbs in his fights with Holmes, Cooney and Tangstad. He had good stamina, great skills, and was a well rounded fighter. He was also younger than the average top rater of today at age 28 when he beat Holmes. For Michael to outbox and take a decision over someone like Maskaev is not out of the question, and the same holds true for most of the other guys that I mentioned. Wlad and Valuev are the only ones who really standout as a major problem in my opinion.
IF Spinks goes 50-50 with a past prime Holmes he beats most of the top 10 today. The pedestrian based Chagaev and the none punching Ibragimov would lose to Spinks. I wouldn't count him out against anyone except maybe Wlad
