My best idea is gently using a razor scraper. I have used them to remove paint and baked on crap on the inside and outside of a kettle. Just be careful about the angle so you aren't driving the blade into the porcelain. If there is some small remnants you can try dish soap and 0000 steel wool.
In the future I would grab a crappy black kettle with no plastic for the kettle pizza. The vents don't even need to work as you just leave them wide open to cook.
Kettle pizzas are hard as hell on the porcelain and any plastic. I use a black premium kettle that I picked up on clearance one year. You have to remember that you have to be in the 600 degree area for temp most times but I have even pushed 700+ (maybe even 800+) when I need to cook pizza fast and in quantity. That is hard on anything.
You could also look into a baking steel for the kettle pizza as it will deflect the heat down instead of onto the lid and vent. It has advantages to cooking the top of the pizza too.
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