Hagler would quit after a round from weakness from cutting weight and chasing Floyd around the ring. Newsflash, Hagler never fought at 154 lbs.
Hagler could make 154 under modern weigh-in rules. He'd beat the cr*p out of Mayweather, which is why the fight would never happen.
It would make for an interesting fight. Mayweather would run and potshot Hagler until about the 3rd round, then all of a sudden after getting hit with one of Hagler's shots, he'd stop and as Hagler approached, he would notice that the canvass was wet. The ref, Hagler and the fans would be wondering what was going on, only to discover that Mayweather ****ed himself...lol Btw, I just finished watching Hagler against Mugabi, what a great fight, and the 6th round is a classic.
The only question is how weakened Hagler is from making 154. Both are ATG fighters, but there is a more than a slight or even mild difference in size. Mayweather wasn't even the same fighter at 147 as he was at lower weights, and he's going up against one of the best middleweights ever. Hagler kicks his ass. If they were naturally the same size, then that is one great matchup.
I seriously doubt Hagler would cut weight regardless of whatever standards we go by in any era, and Mayweather was past his prime for any fight he ever had at 154.. Some fantasy fights just aren't practical.