I recommend contacting a plumbing/heating contractor in your area that can advise you on repairs, especially since you stated the equipment is very old. It is possible that they can repair the problem but it is also possible they will need to replace it if they can't find compatible parts.
When it comes to natural gas fired appliances, I usually leave repairs to the experts.
You could also try and find the manual by searching for the manufacturer using the model and serial number of the appliance to at least get an idea of the parts that make up the boiler.