I've pulled a few thousand shots with my trusty portafilter pictured below. It's got about 7 years of service with weekly cleanings in urn cleaner. The bakelite eventually cracked apart--exposing the severely rusted threaded shaft. I purchased a whole new portafilter (and replacement bakelite handle) prior to messing with this one, but would like to make a naked portafilter out of this one if it can be salvaged.
Obviously, the chrome & brass are more fragile than the shaft.
Thoughts ?

Have you checked to see whether it will thread onto the replacement handle straight and tight? If not, you may have your answer. You might be able to brush out the threads using a soft brass or stiff nylon brush.

If that's a brass P/F you should be able to unscrew the rusted steel threaded shaft. I'm guessing that the threaded stud is normally part of the handle??

Yes, the rusty part ideally threads into the handle. But it wouldn't go into the new handle.

But, Success!!

I resigned myself that I was going to have to crush what was left of the old threads with Vice Grips & used some Liquid Wrench(mmm...tasty!)
Got the old one out & then did some measuring and Internet searching. It's 12mm wide with 1.75mm pitch (space between grooves). A trip to the hardware store, some hack sawing & it worked. Now I can work on the making it naked part.

That is a common problem. To avoid it, pick up a stainless steel stud at the hardware store. To make your portafilter bottomless, use a hole saw, vice and drill press on a low speed. Takes about 2 min, then file the sharp edges off the bottom of the portafilter. I mill the bottoms out all the time.

How To Make Your Own Naked Portafilter

Glad yours came out!

I would recommend something other than the standard, galvanized, steel stud.

Stainless would probably work, as recommended.
Brass would be my personal preference since the portafilter is also brass and this will avoid any galvanic reaction that the steel would have.

I bought a unit in need of rebuilding and couldn't get the bolt out. I tried everything... Finally had to dissolve it out using an alum solution in a simmering pot. Took a few days, but the steel bolt dissolved right out. Rust was literally bonded with the brass until the last 1/4" long piece.

Temporarily I'm using a galvanized bolt as a handle (the industrial look), and before fitting an o-ring and lock-nut to keep the water out of the threads it would rust almost immediately.