Is there a way to default restore the Basic-Decks?
Solution 1:
Yes, there is.
Go here and just copy the hash, when you go to make the deck it will automatically ask you if you want to build the deck from the clipboard, then it's done.
Solution 2:
No, currently there's no way to rebuild the basic decks. With patch 4.2 the 9 additional deck slots we're getting are taking the place of the old basic decks. Once you remove the old decks, they're gone, so if you want to play again with them you'll have to keep them saved or write the cards up to rebuild them in the future.