New gas tank

Siphon out the old fuel and run it through a rag/screen/filter to get any particulates out. Or just buy new fuel for the new tank.

Make sure the seal is okay to use for the tank-to-filler neck section. Might be worth getting a new one. Somebody here on the forum makes them, but I can't remember who.

Make sure your sending unit is good and clean, and there's no crud that might fall in the new tank (i'd really recommend putting a new sending unit in while you're putting in a new tank- they're not too expensive and it's way easier to do it all at once.

Don't forget the ground strap for the fuel gauge to work properly.