Lawn Tractor Engine Problem

I have a 6 y.o. Troybilt riding mower with a 19 hp B&S engine. When I try to start it, it wouldn't start but was able to jump start. However, it runs very rough with lots of smoke for a few seconds and then stops. I can repeat the same thing again.
It seemed fine the last time I used it a week ago. I do change oil (synthetic) and filter at end of every season, and I have only 1/2 acre to cut once a week. What could be the problem? Is the engine blown? I never changed the spark plug but it doesn't look like a spark plug problem does it?
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panabiker wrote the following:

It has a combination alternator/starter. Sounds like the alternator part has gone bad and the battery is not getting a charge. You can jump start it, but won't stay running because the battery is not charged. You can put a charger on the battery and that might help it keep running until the battery gores dead again, but you'll have to change the alternator/starter part.
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Bill
In Hamptonburgh, NY
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