Replace it! 'cuz the first leak in any metal waste tank is always only the FIRST leak. Urine is so corrosive that it turns metal tanks--even 316 stainless--into colanders.
Ronco Plastics (no relation to the VegoMatic Ronco)
Ronco Plastics marine Tanks is your best source for a tank. They make TOP quality thick-walled water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank.
And as long as you're replacing the tank, this would be the time to upgrade the tank vent to 1" and maybe even find a better location for it if it's much further than 5-6' from the toilet. I'll be glad to help you figure out the best way to do it...and you'll also find a whole bunch of good info in my book (see link in my signature). The title (my publisher's idea) is a bit misleading...'cuz although it does deal with every source of odor on a boat and how to cure, or better yet PREVENT 'em, it's actually a comprehensive "marine toilets and sanitation systems 101" manual that explains the laws, describes all the types of systems and how they work, and will help you learn how to install, operate and maintain your system to prevent 99% of problems instead of having to cure 'em. 'Cuz you get to do any preventive maintenance on your terms when it's convenient...the need to cure a problem never happens when it is! And I'm always glad to answer any questions it doesn't.
--Peggie