I was taught a technique for dissassembling stuff that was “oxided together” by a fellow named Keith Hamilton, well respected amongst Velocette owners in Oztralia. Sadly passed away a few years ago.
It is basically heating the component to about 130 degrees Celcius and dumping it in cold water until it is cold and seeing if it moves. If it moves leave it in the water and work it until it frees up, which will not be long. If it does not move, do it again and repeat until it does. The theory is that dry oxide is full of air, and heating it up and dumping in cold water means the oxide sucks in water. Oxide full of water is mush.
My first experience of this was a 1925 B.S.A 350 frame steering head locknut, the usual domed head thing. It was rusted through to the threads on one side and the hex was only a vague outline. After about 4 heatings and dunkings it started to move and leaving it in the water the oxide washed out and I unscrewed it. The steering head thread is undamaged and as a locknut the nut is still useable, although the only spanner to fit it is the multigrips.