Only 750 miles on scooter. Put away with stabilizer. Starts bar-o-o-m like it's going to run fine. Then stops and won't start. Checked plug and spark. Plugged line?

The old gas thing isn't good, but this seems pretty extreme unless the gas was visibly bad. Did you examine or filter it when you siphoned it to be sure it had no visible water or contaminants? Anyway, its certainly worth exploring before going farther. Make sure its got clean fresh fuel. Otherwise, maybe timing belt / chain trouble?

Take your car to the mechanic right away.

PROBABLY NEEDS A TUNE UP AND NEW VALVE COVER GASKETS, SOUNDS LIKE OIL IS LEAKING INTO THE SPARK PLUG TUBES AND CAUSING THE MISFIRE

Have u checked spark plugs.?

Can you jumper the fuel pump and make it start and continue to run ?
Have you checked to see if you are getting spark when it won't start ?
Could be a relay or coil, ignition module, or other component.

If it is a 3.8 engine change the crank sensor,they go bad on the 3.8 engines.

Check for vacuum leak at intake manifold or bad fuel pump relay or have fuel pressure checked to see if fuel pump is working properly 40-45 psi it should be pumping

What a funny set of events.Of course the tail pipe is clear(you would have checked that)The only other thing it might be is the cat/converter is blocked.New muffler is allowing a heap of air through and it suddenly clears and the bang.They do block up so it may be worth getting it checked.Hope this helps

check the spark plug wires are attached all the way till it snaps, pull each spark plugs, any black deposits on any plug means it isn't firing(faulty plug,or wire) last of all take off each injector wire and notice if there is a change in idle speed (plugged injector)

have fuel pump checked or fuel filter replaced