Do you not live in Tulsa and you're travelling here and get to visit the Guthrie Center? I don't know what you mean that they have a new roof and restaurant. The Woody Guthrie Center is just a museum without a rooftop access for the public and no restaurant.
I live in Tulsa - and haven't had the chance to get to the local one yet, which is really irksome. Life gets in the way sometimes. And travel will give me the chance to get to the original Guthrie Center first....go figure. And they serve free lunch every Wednesday!! Strange how it works out sometimes. Also the scene of one of my favorite movies... Just love Officer Obie!!
As for the restaurant, well just walk right in, it's around the back...just a half a mile from the railroad track!
https://guthriecenter.org/