Peugeot with I.E. dies on downhill and at traffic light

Hello, everyone. I own a 2019 Peugeot Kisbee 50cc Euro 4 scooter with electronic injection. Unfortunately, there's no one in my city who can maintain Peugeot scooters, so I've had to take care of everything myself. While replacing the spark plug, I accidentally damaged the spark plug thread.

After getting the head back , I reinstalled the old head gasket, which is made of metal. However, I filled it with oil (idk if it is good or bad). I also did the timing. Since then, I've been struggling to make the scooter run as smoothly as it did before.

I've created a video (click on this text to see video) [1:46] that provides a detailed explanation of what's been happening so that you can better understand the situation.

I attempted to adjust the throttle position sensor and, during my testing, I would consistently clean the oxygen sensor. This allowed me to monitor whether the fuel mixture was running rich or lean. But still the same thing happens everytime, when fuel rich it has better idle but spark plug and ox sensor would be so dirty

I must say that scooter had some random missfires before my attempt to fix it and this scooter has only 8000km. I've been driving the scooter in this condition for almost 100 kilometers, and there doesn't appear to be any oil leakage.

Thanks to everyone who is willing to assist in diagnosing this scooter.