You might consider having a metal shop replace it. In thin sheets of metal, rust compromises the material's strength rather quickly. You'll want that support structure to be as rigid and strong as you can get it if you plan on this tool being useful.
If you believe it's salvageable, I find that a combination of abrasion and chemistry is best.
With full ventilation and while wearing a respirator and eye protection, use a wire wheel on your favorite high-power rotary tool to clean down to bare, shiny metal. You may need a grinder wheel if the rust is as bad as you say. Once you're down to bare metal, assess the thickness of the remaining steel and see whether this is really worth your time.
When you have brushed it well, apply naval jelly (phosphoric acid in a gel emulsion, available from any hardware store) according to the label directions. This will remove most of the lingering rust and chemically convert the microscopic remainder into a non-infectious form.
Immediately prime the metal with a primer specially formulated for steel and let it dry well. Then you can apply enamel or whatever you want as a final finish.