Cleaning a Stained Toilet?

I have a toilet in a vacant rental house that was used many times (by someone who broke in) after the water was turned off, so basically it was never flushed and full of nasty. I have cleaned it numerous times.

It was a new toilet, a $400 low water energy efficient one. I cannot afford to replace it. The stains (brown) are not from hard water, as there was no water in it. The "solid" material is out, but how do I get it white again? I have used every cleaner, bleach, and vinegar I have and no luck.

By mom-from-missouri