How do we know that the book of Enoch was translated correctly, are any of the original writings still intact? Or were the supposed "originals" found already translated. I ask this because some valuable words can be "lost in translation" such as the commandment "Thou shall not kill" or "Thou shall not murder", which can and has been interpreted differently throughout history. Another example would be some Christians believing in "Hell", and others believing in the "abyss", and again others believing that the two are separate, one being a place of holding and the other following eternal damnation by god's judgement.
To directly answer your question Wikipedia dates the oldest copy of the book of Enoch as being from 300BC. Parts of it like the section called the Book of Parables were not found in the Qumran.
I agree with AffableGeek. If we were to go back in time and mystically derive from Plato a perfect translation of the Republic it would not be canonical. If we were to modify it and cross reference it with Bible verses and give it the name of a supposed Saint it still would be Platonism. If the Book of Enoch was predestined to be used by God as canon it would have been re-written by Moses or one of the other ancient inspired writers very similar to the Book of Job and and we would have it available just as if we had traveled back in time and heard the heart of God concerning the topics contained therein. However the Book of Enoch was not deemed fit enough to be rewritten and preserved by God through Moses (or other inspired writers) so we know that it failed in some (if not all) crucial areas. As per the supposed two sentences referencing it in the New Testament, like Plato, having some similarity or parallel to a Bible truth expressed in inspired writings doesn't validate the whole.