36 years old female non smoker. is it safe to fly overseas with a basilar artery fenestration? no other health problems?

Basilar artery fenestration is a congenital anomaly typically of no significance, but rare strokes have been attributed to its presence. There does not appear to be any risk secondary to international flights, due to presence of pressurized cabins adjusted usually to no higher altitude than 5,280 feet. (Do check, however, whether your destination is a COVID-19 "hot-spot".)