Being so old of machine, The one place and guy I can think of to help you is in Michigan.
Try this Espresso Parts or https://www.espressoparts.com/
They were built originally in NYC by Rotlevi. Who sold the idea to Grindmaster. The only US expresso machine. But a lot of the parts were still the same as the Italian machines. Since Grindmaster and Cecilware merged they dropped all support and parts for it.
By in far the most common one is this;https://www.espressoparts.com/sirai-pressurestat
Retrieving data ...