First check either the tube sheets, heads or the adjoining vessel surfaces of the pressure vessel for possible stamping about the identity of the pressure vessel. The stamping may be hard to see due to the age of the vessel so one trick which I learned as a former NB of PV's is to moisturize the suspected stamping areas that I mentioned with a wet rag and then shine a light from a flashlight to highlight the markings. If the vessel is insulated some portions of that insulation may have to be removed. Since you did not state what type of vessel you have, the markings will either be on the upper center portions of the tube sheets or close to the shell edge welded to one of the tube sheets,
About your question in your OP, it is very unlikely that your PV will be accepted by an authorized (NB/ASME) boiler and pressure vessel inspector, however, there could be a way out but first talk to an insurance company that provides boiler and machinery coverage for guidance from their specialist. for a fee, that specialist may perform the required evaluation and all associated calculations or may ask for an evaluation from a professional engineer specializing in fired and unfired pressure. If that report shows consistency with the applicable ASME code then you have a shot at the vessel being accepted by the B&M authority of your State. The State may even assign their own number to your installation. All requirements set by the National Bord (NB) must be met for a NB number to be assigned to that vessel.
Before proceeding you should also make inquiries to the Nation Bord of Fired and Unfired pressure vessels. If you contact the State for guidance you may be required not be able to start until you meet that State requirement.
