Replace it!!!!!
It is best to replace the head if you can not fix or find another valve. If by chance you do find a way to glue the pieces together, it will break again. Heads start at around $150.00 and up. You really do not need anything fancy. A head is a head even with all of the bells and whistles. You will need to pump and flush all sewage tanks and lines before any repairs or replacements and it will still be a smelly job. Make sure you wear rubber gloves, clean up the head area after the parts are removed, put the head in, then take a shower. While working on the head, refrain from scratching an itch on you face and eyes and use a cloth to wipe the sweat from you face and eyes.It also helps to have a helper retreive and open your water/beer as you work, to prevent you from tracking the wrong stuff all over your boat.