I have an ~5 year old Simplicity tractor with a Kohler CV 460 engine. End of last season, I drained and ran out the rest of the gas (as I do for all my gas powered tools at the end of the season), and changed the oil. Went to start it up this year and nothing. Starter motor can't turn it (the starter motor works "on the bench;" I also tried running it off a running-car battery); I can rotate the shaft with a breaker bar or ratchet, but barely, regardless of whether the plug is in or not. It feels like it desperately needs lubrication, but the oil level has always been where it's supposed to be--and, as I said, it was fine the end of last year. I've stripped everything off, including the flywheel to no avail. As a probably irrelevant aside, I never thought the engine was putting out quite the torque it should have, but I really never had anything to compare it with. Really don't want to go in if I don't have to--I'd appreciate any suggestions.

I can rotate the shaft with a breaker bar or ratchet,

Is that the starter shaft or the engine flywheel shaft.
Just checking on shaft lingo here.

This could be a stuck valve or a cracked ring.

Look in the cylinders for scrapes/gouges in the walls, or any metal pieces.

Also check to see if the mower blades are disengaged and you aren't trying to turn the whole works.

A frozen engine basics are this
Cracked rings
Push rod broken
Connecting rod spun on bearing and hits valve
Valves bent

Flywheel shaft--starter appears fine (works when disengaged)
I've stripped all loads from the motor--even the flywheel's off

I should add, or reiterate, the shaft does turn, and it feels the same all the way around. It's just that there's (a lot of) constant resistance.