Slowly add regular white vinegar to the water in the bowl, it may take a lot of vinegar so you will want the cheaper store brand in a gallon jug. If the lime scale is above the water/vinegar you can slowly pour in more vinegar & water to reach the lime deposits. Or try saturating a rag with vinegar and apply to the deposits and leave it for an hour or so. Check back to see if it's working or needs more vinegar.
I would try using a product like CLR or even white vinegar in the bowl. I would shut off the water to the tank, then flush toilet, and use a plunger to remove as much water as possible. Put in the cleaner, let it soak, and then use a sponge with a scratch pad to break up the stain.
Lysol Lime & Rust Remover Toilet Bowl Cleaner and Mr. Clean Magic Eraser. It worked better for me than bleach and pumice stone.
The Limeaway that is in a squirt bottle, out in and saturate, let sit for an hour and then an old fashioned pumice stone and elbow grease. Depending on how much there is you might have to do up to 3 times until it is all gone. Them I use Bar Keepers friend toilet bowl cleaner to make sure it is super clean with a Magic Eraser. I am professional cleaner and works every time.