Unable to start mower

I have a Craftsman riding mower with Kohler CV22 engine. A couple of weeks ago I put a tank of old gas in mixed with some new. It was not running smooth but kept running all the same. About halfway through the mow I noticed a lot of oil in the fuel filter. Uh oh. I drained the tank until I could see no more oil coming out, replaced the fuel filter and finished the mow. The oil was very sticky.

This weekend it wouldn’t crank. I happened to have a spare carburetor so replaced that rather than try to get the sticky oil out of the jets. Carb seems to work fine; there is fuel in the bowl and the solenoid that lets fuel flow works correctly. I have been trying to crank it for two days via regular fuel feed from the carb, then fresh gas from a bottle, then starter fluid. I haven’t heard it try to fire the whole time. Removed spark plugs and don’t smell gas on them. I’d think they would be swimming by now. Is it possible the valves have gummed up and aren’t opening? I’d think there would be some major mechanical clanking if that was the case, but I don’t know. Any suggestions?