I work as a sound engineer in a radio station, cleaning up hundreds of noisy, outdoor interviews.
It would be really helpful to hear at least a sample of your file before I can tell you a fair percentage concerning the amount of restoration possible in your case.
I might not be a "miracle worker", but I am a pro in my field and I can deliver "clearly hearable sound".
Hoping for a successful collaboration,
Regards,
Adrian Faur