Craftsman LT1500 Riding Mower won't stay started & backfires through carburetor

Help please! I have a Craftsman LT1500 Riding Mower Model #247.288810 and it was running just fine... broke a Upper Drive Belt. And other Lower Drive Belt was in bad shape. So I had a small engine mechanic replace both belts. When I got it back - it was running kind of rough, then it quickly overheated and I shut it off, put it in the garage. Let it cool off overnight and the next afternoon - It acted like it wanted to start but it would backfire through the carburetor and died. Someone said to try the fuel cap and tried that... no luck. Thought maybe it could be water in the gas - drained gas and put in fresh gas. Still backfires through the carburetor and dies. Replaced spark plug, and the battery. Still the same thing. When I turn the key to start it it acts like it doesn't want to turn over. I turn it back to the off position and try to turn it over and it will try to start it, it starts for a few seconds roughly and then it backfires through the carburetor and dies. Could the mechanic have done something to cause this issue or what could be the problem? How can I fix it? Thanks

 

It sort of sounds like a sheared flywhel key. I'd check the carburetor bowl for trash first though.