Yes, I used the old gasket. I ran into the same situation. I had order gaskets for the TORs because I didn't realise new gaskets came with them. I missed realizing I needed a gasket for the header pipe. I used a small thin flat screwdriver that I ground wafer thin to separate the edge of the gasket from the shell of old pipe. I gradually did this a little at a time as I worked my way around the circumference of the pipe and gasket. I also saturated the separating area with penetrating oil as I did this. Using the screwdriver and oil allowed me to separate it from where it was sticking to the interior surface of the pipe.
Take your time and go slow. Anyway, it finally came out with very little damage and was completely intact. Had I notice the gasket on Hermy's website as I looked at the diagram, I would have ordered one. I will order a new one for use in the future, but the old one seems to work just fine. Except for the smell of the penetrating oil burning off after startup, all was good.