For any mask, a crack is the step before a serious tear. This doesn't mean that it's not still a keeper, it only means it should be placed on a stand and displayed.
To fix a crack, use strips of 1/4 inch masking tape, and tape the hole together. From the outside, using the tape only to keep the sides located.
From the inside of the mask, spread yellow contact cement around the crack and about 1/2 inch on either side. Allow this to dry.
Cut a strip of paper towel, cheesecloth, or muslin as wide and long as the glued area inside the mask. Lay this on clean newspaper, and paint yellow contact cement on one side of the paper towel, cheesecloth, or muslin strip. Let this dry. Carefully lay the glued paper towel, cheesecloth, or muslin onto the glued crack. Press firmly and remove tape. Brush the paper towel, cheesecloth, or muslin patch with more contact cement. When dry, dust with baby powder.
Now place the mask on a display stand, and enjoy.
This may not be completely perfect, but it will help keep your treasured mask together.