Rip it out, plug it up, put in a porta potty.
Or a five gallon bucket and thick plastic bags!!
I would rip it out, plug it up but definitely not put in a Porta Potty. Those things stink to high heaven.
I have been using my homemade composting head for six months now and I give you my word that it is simpler, cheaper, cleaner, less smelly and safer than a regular MSD.
It doesn't need to be any more complicated than a five gallon bucket and some sawdust.
Read this online book if you have time. It's worth it. The Humanure Handbook - Table of Contents
I built two units. One for the boat and one for my home. I would never go back to using a water wasting toilet in my home and certainly would never go back to having to carry around a stinking cesspool of sewage on my boat.