Plumber?

I have the original Rudd continuous feed water heater from when my house was built (1928!). It’s a beautiful thing—ornate cast iron with a series of copper coils through which the water flows and is heated. Most of the time it works great but every now and then it seems as though the water doesn’t circulate. Even though the pilot is lit, we have no hot water. The problem inevitably fixes itself in short order but I’m concerned that it might need some servicing (hasn’t had any in the 3 years I’ve lived here). I need a plumber who understands old things. I do not want to replace it unless absolutely necessary. Any suggestions of who to call appreciated. Elisabeth