Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

I would suggest electrolysis as a means to remove the rust.  When you're down to bare metal you can re-blue the boiler.

There are a number of how-to videos on YouTube.  Here's a quick demo: https://www.youtube.com/shorts/RoN1CN1Iy8k

Here is are a few more detailed videos:





Best of luck!

Paul