This engine on my Scotts lawn tractor stopped suddenly, died, and now will not rotate past a certain point. I suspect something has come loose or broken inside. Any clues as to what the problem might be?

Hi,
If it absolutely refuses to rotate past a certain point, it has most likely thrown a rod(connecting rod.) This would most cheaply be solved by cleaning the crank journal and installing a new rod. This is also the slowest way. The quickest way is a new engine, and in between is a short block.
Peace,
Clarke

Thanks, Clarke. All of the options sound expensive. I will consult with my mower mechanic and decide what to do. A new mower may be in my future.
Joseph