Hello, such an old boiler needs a close inspection by one of the local *industrial boiler service companies (not a residential HVAC shop.) Very often the less you do with an old boiler the better off you will be. some types need pressure vessel certification and regular inspections.
If the gaskets do not leak, they should be left alone, if fire brick is in tact it should be left alone, possibly re-pointed if the mortar has fallen out in spots. That needs to be seen however.
All of the fire controls need to be inspected and tested once every month or so, generally at least twice a year on contract with an industrial boiler service company, depending on the size of the boiler and its application.
If you can send me a few well lit pictures of the inside of the fire box, and the boiler room itself and the boiler taken from about 15 feet away that will help... I need to the application, is hot water, slow pressure steam or a high pressure steam boiler.
Does it burn natural gas or oil?