Toilet causes sewer smell

I inherited the toilet as part of a house I purchased. The seat was loose because seat bolt holes are much too large, even for normal bushings.

Giant problem however is the sewer (urine) smell that occurs. The odor seems to come up from the drain hole at the bottom of the tank. For several months I assumed the slight odor was from messy toilet habits, but today I took the tank top off and removed the tray of water. The urine smell is now pervasive even though I have dumped several gallons of water into the tank.