Yes, I think it will fit. If not, then the port size on the cylinder is about the same for all the small singles so get an adapter for 325 engine. But I don't think just changing the carb will help performance. You will need to do porting and polishing along with exhaust to match and probably fiddle with the jetting to get it to run right. Don't get me wrong, there are a lot of things you can do to get more performance from your 250. But you have to address the whole system not just one part. An engine is just a big air pump. So if you increase the size of the inlet without doing something to get the air through and back out you won't gain anything.
Mine: 2018 Sportsman 850SP - 26" Bear Claw HTRs on OEM steel wheels(I really don't like the Aluminum ones)
Polaris Hand warmers, Mirrors.
Hers: 2007 Sportsman 500 EFI, Polaris Winch, 26" Bear Claw HTRs on Moose 427x Wheels. QSC Clutch Kit, Hand Warmers
2001 Magnum 325 MOSE 12" rear wheel conversion. 25" Sedona Coyote tires
2001 Magnum 325 Warn winch and Moose 60" plow
2017 Polaris Ranger 1000XP - Roof and front glass
SOLD 2004 Sportsman 600 Twin