Best bet get a flush king to try the cheap way. It should help to get things moving again. Then when you have space to fill the tank with water and get the mass softed which may take some time. If you have a straight drop from the potty to the tank you may be able to break up the mass with broom handle to aid in flushing. If you do this make sure that what you use is not sharp you wouldn't want to poke a hole in the bottom of the tank. I'm sure there will be a number of replies with other suggentions of how to drain the tank. Just remember the stink came from inside you.