If it's like the other older machines the rear is a 10" wheel with a 4/4 bolt pattern. I do believe that Delta makes a 12" wheel that will work. I believe the last ones I saw were about $125 for a pair. I can not say what a set of those wheels will do for the wheel track. I have a '94 Trail Boss 400L in the shop. If I remember it I will look at the wheels tomorrow.
With my 325 Magnum with the 4/4 bolt pattern I wanted to go with a newer style 12" wheel. My research showed the adapters had to make up for about 5" of width difference also. So I got CAD drawings from another member of the forum. I have a tool and die maker friend who had too much time on his hands and a CNC end mill. He got the material and we made 2 sets of adapters.
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: 2019 Sportsman 450. 26" Bear Claw HTRs on Moose 427X Wheels.
2007 Sportsman 500 EFI, Polaris Winch, 25" Bear Claws QSC Clutch Kit, Hand Warmers
2001 Magnum 325 MOSE 12" rear wheel conversion. 25" Sedona Coyote tires
2017 Polaris Ranger 1000XP - Roof and front glass
2018 Kawasaki KLR650
2001 Magnum 325 Warn winch and Moose 60" plow-Sent to Daughter and Son-In-Law since his blew up
SOLD 2004 Sportsman 600 Twin