How can I get unknown part messages off settings menu?

The way to get the message off the phone is install the correct battery in the phone. Otherwise, the message will probably always remain.

If you are lucky, you might still be able to check the Battery Health. Settings > Battery > Battery Health > Maximum Capacity. In many cases when the wrong battery is installed, the Battery Health settings disappear.

Unfortunately, since the phone has been tampered with, Apple will likely refuse to even look at the phone if you take it to them.

I don't think you can. See:

https://discussions.apple.com/thread/253634059

Hello ~ That message will stay there. One of the reasons to get your battery replaced at Apple.

~Katana-San~