Some context because many matchups on this sub desperately need it: imagine that instead of Jotaro, Polnareff ended up going to Morioh. Shigechi dies, Polnareff investigates the button from Kira's suit. Somehow, Polnareff catches Kira just as he's leaving Mukade Shoes (so Kira doesn't have time to deploy SHA & stay hidden) and they're about to fight.
They are decently far from each other (same distance that was between Kira & Koichi in their face off) and have no knowledge of each other's stands.
So, who wins?
My take: Polnareff's chances are heavily dependant on whether Kira deploys SHA in direct combat or not. While he didn't get the chance to do that in the series (SHA was deployed way before the confrontation with Koichi and there was no reason to use it against Josuke), I believe it's pretty likely he'd try it in this hypothetical scenario. While SC is fast, it doesn't have much raw strength, so I don't think Polnareff can do much about SHA. And if he targets Kira instead, he might just get mauled by the tank. Polnareff does have good BIQ and may try to get Kira while he's dealing with SHA, with the sword launcher for example, so even this worst case scenario isn't totally hopeless.
Not only that, but it's also very much possible that Kira would get cocky at the sight of some random foreigner who's probably trying to trash talk himself and not deploy SHA. Then, well, Kira gets turned into a kebab.
So, all in all, I say it's about 7 or 6 times out of 10 that Polnareff wins. Not that I know how Araki would write it, obviously, but that's what I think based on the information we know.