Since I am not familiar with the exhaust pipe retention...Is this the same as old Brit motorcycles where a threaded locknut end presses on a formed flange of the exhaust pipe and holds it to the head?? If so with the lock ring gone there is a smooth pipe remnant inside a threaded lockring shell still holding the pipe to the head? So corroded threads are still the main problem?
If this blather is somewhat correct ... Turn machine so exhaust is up. Apply drops of (your favorite) liquid wrench to thread area and let it sit . Now I think of an "inside pipe wrench" which I believe Harbor Freight sells. They are toothed, expanding metal rings made to bite in a counter clockwise direction. But IF one fits you may find it might loosen the pipe so it spins inside the threaded locknut shell, but won't come out. Then perhaps force something down between the loosened pipe and the threaded stub and pry it into a smaller dia so it can be removed. Then work on the threaded shell with the inside pipe removal wrench.
All armchair theory and can be ignored at any time.:laughing:
Edit: HF does not show the wrenchs, but General Tool and Rigid have them out to 2.0" One youtube dude used an impact wrench on the tool.