Personally, I would completely disassemble the kettle and give the bowl and lid a thorough cleaning. I use a razer scrapper to get a majority of the gunk off of it and then use 0000 steel wool and dish soap to scrub it completely clean. If there is still some rust and it isn't too badly rusted you can try using some evaporust to get the rest of it. There is even a chance that you might need to let it soak in evaporust just to get the one touch sweeps off. They might also be cut off with a cut off wheel or grinder, but you need to be careful. If a kettle is really badly rusted though sometimes the rust is all that's holding it together and evaporust can eat it away too much. Once it is all clean I would use cooking oil on any bare metal and cook on it to season it. I would try to clean up the handles as best as possible because that is the one part of the kettle you can't easily replace.
As for the lower part of the kettle it is likely going to be more work than it is worth to clean up. It can be done, but I wouldn't doubt if you could find a black kettle from the same age for $20 to $50 in much better shape. The sweeps are shot so throw them away for replacements, the legs and the bucket could be polished up with some emery cloth sand paper, the triangle could be soaked evaporust and repainted/clearcoated, and the wheels could get replacement rings. That is just a hell of a lot of work and a fair amount of materials in my opinion. You can always use the materials again for the next project though. Right now I have a few buckets of evaporust that I bought months ago thinking I would find a few good projects this year, but now I am stuck at home with all this madness going on.
Good luck!
