Could Spinks compete on a level field against the bigger guys from the 90 s onwards? Guys such as Lennox,Bowe, Klitschko s etc?
None of the guys you mentioned. They were a bridge too far for Spinks. But, there are countless belt holders like Maskaev, Rahman, Ruiz, Charr, Parker, Browne, Martin, Stiverne etc that would likely lose to Spinks.
I honestly think he would even beat a guy like Wilder. Wilder would have to land a lucky punch to have any chance.
Yes. A lot of guys from the smaller weight classes could compete with HW's. But competing(having a healthy chance to win a fight against most opponents)is not viable. Fighters need to win almost every fight and if they can't do that they don't go to HW. But to your specific question about Klitschko, Bowe and Lennox the answer is no. Of course not most of the best HW's didn't compete with them.
Moorer did it, Adamek did it, so i dont see why not as he was better than both. Would he beat all the elite behemoths? Hard to say. But he could definitely have success in the division and win a paper title or two.
Probably not the elite of the big nineties men. But I think there were quite a few second tier guys he could have beaten. Snatching the WBO title from Ray Mercer is one strong possibility.
Sure don't like the wear and tear factor facing that crop of guys. Just think that older version of Mike and those knees would just not be good enough for his herky jerky movements he'd need against this level of competition.