Hi Kristina,
If your father was born in Romania, then I would say that his name was actually Stefan Coman (this would be the Romanian equivalent for Steven Koman). If you know the date and city/town of birth, try to look by the Romanian name - maybe contact the Registry of Births (in Romania it's called 'Starea Civila'). Also, 'Julia' would be 'Iulia' in Romanian, and 'Leah'='Lia'
Good luck!
M