Let's eliminate Mac from the equation because, as far as we know, he played on roids pretty much his whole career. We simply can't know what he'd have done without them.
Then there's the question of A-Rod. Was he juicing in 2001 and 2002? And if he wasn't and he got to face a league full of pitchers who weren't, would he have beaten out Maris?
Guys like Jim Thome, Shawn Green (49 HR in 2001) and Jr. possibly could have made serious runs if they were clean and if they got to face a league full of clean pitchers.
Then there's the two leading candidates: Bonds and Sosa.
Could Bonds have done it without becoming Gigantabarry? It's important to note that Bonds broke the record in 2001 partially because he decided to. He basically played that season with the attitude of "Oh, you want homers? Then I'll give you homers." His homer totals were his way of flipping everyone the bird.
At his core, Bonds is a baseball purist. Once he got the single season HR record, he went back to being the most ridiculous all-around hitter he could be. Could he have hit 80 HR in 2002, 2003 or 2004? Maybe, but he went back to his default wiring.
So could an angry, motivated, unenhanced Barry Bonds have topped 61 HR? I think there's a decent chance he could have. He'd have put that loft on his swing and maybe the balls fall in the stands instead of flying into the bay, but they're still homers. Dude wanted to make a point and he very well might have been able to make it without the juice. Bet he wishes now he'd tried it that way.
Then there's the curious case of Sammy Sosa. He had pretty good power before 1998 and his power spike came at ages 29-33, which is fairly standard stuff. He was athletic, he had a fast bat and he had a fairly powerful build (still does, unlike Bagwell Sammy's still pretty much the same size).
Like I mentioned above, if you had to pick a breakout HR guy from a list of likely candidates prior to 1998, Sosa would have been a decent pick.
He also started doing something different at the plate in 1998: taking pitches. His BB rate jumped to 10.1% from 6.3% and he climbed over 4 pitches per plate appearance for the first time in his career. He cut the number of pitches he swung at by 6%. He literally saw dozens more 2-0 and 3-1 counts. Regardless of whether he bellied up to the juice bar, dude figured out how to hit.
Mix Sosa's established power and ability with a vastly improved plate approach and put him in a league populated with expansion pitchers while he's still in his prime and it's a recipe for a power spike.
And here's a sub-roids question on Sosa: not did he, but if so when did he? Conceivably he could have started in the early 90s. That could have been part of the latent power that got unleashed in '98. Or maybe he started after '98 in order to keep the homers coming. Picking '98 as his only possible starting point with PED actually increases the potential that he achieved something while still clean.
And sub-sub-roids question on Sosa, if he used was it hardcore usage the whole time or was there a period where he only used sparingly? And, if it's the latter, then did he have the nascent ability to beat out Maris without any juice?
Given the vast amount we don't know, the rabbit hole goes pretty deep on this one.