The little paper dust/paint masks they sell in any paint department work well for keeping dirt out of the mouth. They come several in a package and are cheap. Before you have used them all the little rubber strap that holds them on rots so buy as few as you can.
Dust through your goggles doesn't need to be a problem. First the goggles must fit your face, around the nose is usually the problem. Second they must fit inside your helmet properly. If the goggles press against the helmet on the outside edges, it pushes the foam up in the center allowing dust in around the nose.
Goggles don't last too long. The thin foam vents above the eyes wear out first, then the foam around the forehead goes flat. I have never found a satisfactory way to replace them.
If sweat is a problem many guys use very thin panty liners, stuck on to their foreheads. If you don't know what I am talking about ask a woman.
Wayne