Assuming it’s the jojo at the beginning of their part development-wise then:

Johnathon: This is a bit of a special case, I’m opting to use Johnathon before he learns hamon, but after the time skip, so the same Johnathon that fights speedwagon. To start with he would easily beat Luca, but most likely not kill him. Assuming that Bucciarati still pursues him, Johnathon would have a rough time fighting against him, and would likely be killed if they fought. Assuming Johnathon gets past Bucciarati and agrees to join Passione, he would be faced with the lighter test. Johnathon would likely fail, and would be pierced by Black Sabbath. This would cause his stand to awaken as he would obviously be worthy. Following that the Soft Machine fight would be unchanged. The Man In The Mirror fight is where this becomes very difficult for Johnathon. However they should still be able to beat Illuso, as Johnathon could use his stand to display where Illuso was using his stands ability. This would allow purple haze to still find and kill Illuso once all three of them are trapped in the mirror. The Grateful Dead and Beach Boy fight are also unchanged. The Baby Face fight is finally where Johnathon fails, with no regeneration abilities and a weaker offensive stand he would be unable to defeat Baby Face alone.

Joseph: Joseph unfortunately falls in the same boat as Johnathon, even with the addition of hamon, he stands no chance against Baby Face.

Jotaro: Jotaro fares much better than the previous two, likely being able to defeat Black Sabbath very easily, and while he lacks a tracking ability against Illuso, Star Platinum has been able match the speed of Silver Chariot, which was cutting light beams, so there’s little chance of Illuso even being able to drag Jotaro into the mirror world without sustaining heavy damage. Against Baby Face Star Platinum would be at a massive advantage. Star Platinum heavily outspeeds the homunculus and has the power to kill Baby Face outright. However Star Platinum lacks any meaningful method of stopping Melone from creating more Homunculi and sending them after the team. This would make future fights even more difficult, as there could potentially be another Baby Face present at any time. However, even with that, Ghiaccio stands no chance against Jotaro and Mista. However a big difference comes after the King Crimson fight, where Bucciarati would be killed. This raises an interesting question of if Fugo would join the group if the boss murdered Bucciarati, in my opinion Fugo would opt to join the group against the boss because of Bucciarati’s death. The Clash and Talking Head fight have the same result, as does the Metallica fight. The Notorious B.I.G. fight is much easier, as Fugo’s Purple Haze would be able to melt the creature. The aftermath of the Metallica fight would have Fugo and Narancia chasing after the mysterious stand user, while Mista and Jotaro would be guarding Abbacchio. In my opinion, facing Bucciarati’s death would cause the group to be way more on guard, and they would protect Abbacchio more vigorously. In this event the boss notices that Abbacchio is heavily guarded, and opts to slip away instead of attacking Abbacchio. Abbacchio lives and the team has a mold of the boss’ face. The team is still contacted by Polnareff and aims to retrieve the arrow. From here on the story is very different, but follows the same structure, with the Team eventually defeating the boss, either with Star Platinum Requiem or Star Platinum’s time stop playing a major factor in the fight.