shut the water off to the tank, flush, then sponge the remaining water from the bottom of the bowl. Pour in CLR or a comparable product (zep makes one) in the bowl- enough to bring the level over the stain. The last one I did also had 50 years of calcium and lime buildup. It caused the toilet to overflow and not flush properly.
Thanks, Bob. Much appreciated.
Try this : https://www.craftylittlegnome.com/clean-toilet-coke/
I use a scouring stick from Walmart.
You can use a pumice stone.
I’d definitely go Bob’s route as it’s the easiest and most efficient.