From your description I would guess it is probably not top end. I am aware, having bought 2 of them that way, that they can throw the rod bearing out. Usually it is a catastrophic failure that results when the bearing seizes, breaking the rod and sticking the wrist pin out through the crankcase. The last time, one of them spit an oil cooler line off on the trail and trashed the bearing before my son could get it shut down. After we got it back to the truck and filled with oil again I could get it started, but it acted just as you describe. Ran fine and lost power. You can take the head off with the engine in the machine, but you will probably be farther ahead to pull the motor out and tear it down.
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