On that size of boat, I am assuming there are at least 2 other heads. Perhaps this one really isn't necessary? Before you rebuild it, consider the other uses you may have for the space - dedicated shower, wet locker, dry goods storage, etc.
Not saying you shouldn't look at getting it working but maybe it really isn't necessary...
OK, actually looked up your boat and recognized how stupid the above sounds. I wasn't familiar with your boat and ASSUMED it was 66 feet long...
Sorry for the distraction.