How Do I Clean this Old (Sorta) Scary Toilet? -- Good Questions

Pin it Q: I recently moved into a new (to me) place in Brooklyn. It's great, but it definitely needed a deep cleaning before I could unpack. Everything is in pretty good shape now, but there's still one serious offense I'm having trouble with. The toilet in my bathroom has some major issues. There's some sort of nasty hard scale in the bottom of the bowl [photos below--don't worry, they're not horrifying]...