I'm kinda here as well. If I was doing it I'd put a couple rubber orings on that cap and run it. See if it sealed up.Do you really NEED a new tank? I thought it looked pretty good after you worked on it. I would guess a couple of my tanks would leak if I had the tank out and tilted it to the left, but they do not leak when the tractor is in use.
IF you do need a tank, I agree that buying new, with the extra parts included, for less than $200, is a better option than $100 for a used tank.