In most cases it will not be worth even trying to fix your existing toilet.

A toilet gets high use, and is subject to a rough life, and parts are expensive relative to replacement toilet suites.

I guess the old dunny is really just serving a purpose for the residents of the building, however it is really nice to use a clean new toilet rather than one that is 30 yeas old, is badly stained, smells really bad, and just makes you crawl.

The other great thing about a new toilet is they are more efficient and in most cases flush better than the old ones.