Great Grandmothers birth certificate doesn’t exist. Not sure how to proceed…

I qualify for dual Italian/US citizenship. All documents are acquired and in the process of translation and getting apostilled EXCEPT for the birth certificate of my great grandmother. She was born in New Jersey on December 22, 1919 (died in 2015) to Italian citizens. I have a certificate of her marriage to my great grandfather - a Swedish citizen, but married in the US.

I’m not sure where to go from here or what to do. Her son, my grandfather, is in his 80’s and while completely sound in mind and body has no clue any info about his mothers birth (as I’m sure most of us don’t have much detail about our mothers’ births either overall…)

What options do I have? This is where my research skills fall off the deep end.