My son bought a cast iron kettle at a yard sale, nice 20 gallon kettle with a stand for $10. The problem was just like yours, someone had drilled a hole in the bottom and made it into a planter.
First we took the angle grinder and cleaned up the around the hole, both sides, until we had bright shiny metal. Then we took a die grinder with a tapered stone and cleaned up the the inside edges of the hole. We purchased some cast iron rods for the stick welder, shined up a spot on the lip for the ground then started filling the hole. Filled it from both sides then ground down the weld. We put water in the kettle to check for leaks for a couple of hours and found none. Used a cup brush on the angle grinder to clean the kettle, then washed it up and seasoned the cast iron with lard and heat. Been some good chili come out of that yard sale kettle.
Good luck with yours.