My elderly neighbor has an old Cub Cadet 129 tractor with a Kohler 301a 12-hp engine that I use to cut her grass. It has worked pretty well for years until today. I cut for about 30 minutes and the engine started to smoke (light-gray/white smoke). I can't tell exactly where it's coming from, but it's NOT coming out the muffler exhaust.
After a minute or two of smoking fairly badly, it started to run very rough and stalled. I tried to restart it, but the starter couldn't turn the crankshaft. I tried to turn the crankshaft by hand (which ordinarily I can do), but I couldn't turn it either.

I let it sit and cool for a while and tried again... it was difficult, but it finally started up again and seemed to run fine. However, once I throttled the speed up to high for maybe 2 minutes, the smoke, rough running, stalling, and crankshaft "binding" occurred again.

She has very little money for repairs (which is why I cut the grass for her). I can repair most things, but diagnosing this kind of problem is not my strong suit. Does anyone have any ideas or suggestions? She and I would greatly appreciate any help you could provide. Thanks.