I have seen where HJ thinks that this is a great toilet.
Because he knows what he is doing, he can easily and with just a few parts bring them back to life.
You don't need to replace all the parts. You really just need to replace the moving parts. That means you don't need to get ripped off with the installation of a whole new expensive fill valve; just replace the few moving parts in it that need replacement. HJ says that any Briggs dealer can help you. Have a look at the thread SK cited above; it has all of HJ's recommendations, including the names of the parts that you can inexpensively replace.
HJ also, in that thread, tells you what you need to do if you must replace it, because there is no direct replacement.