Hello city of angels, welcome to the forum.
My buddy just got a brand new motor as well and both plugs that come with the kit fouled out within the first week. Replaced with a NGK and it ran so much better.
But the same day we replaced with the NGK it stopped running again. Acted just like a fouled plug. But it turned out the wires from the CDI to mag came unplugged. Bottom line, the stock plugs foul very easily.
It is possible you fouled another plug
Check for spark, Check all wire connections, plug wire into CDI, kill switch button sticking.
Check for fuel, after trying to start it is your plug wet? Possible fuel valve, fuel cap venting problem, gunk in carb ect.
I will do house calls if need be to get you up and running.