Replace the tank
"Corroded" means it's a metal tank...and urine is so corrosive that if it's eaten through the fitting the tank itself is about to turn into a collander.Your best source for a rotomolded tank is Ronco Plastics http://www.ronco-plastics.com (no relation to the VegoMatic Ronco, btw). They make top quality tanks for a very reasonable price and have more than 400 shapes and sizes to choose from, 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.If they don't have one that'll fit, the best source I've found for custom welded plastic tanks is Triple M Plastics. They use polyproplylene instead of polyethylene, but their prices are WAY less than any other custom tank mfr and they put a lifetime warranty on 'em. http://www.triplemplastics.net/You MIGHT want to check out the link below before starting this project.