What should I put on this instead of cheddar cheese? And which sauce instead of sour cream?

Well, it doesn't even call for sour cream. But it sounds good to me without either cheese or sour cream. No need to add either. If you wanted, I guess you could use a vegan cheese & sour cream, but personally, I would probably just add a couple dollops of guacamole and maybe a tiny bit of salsa, depending on how spicy the filling wound up being.