"Do it. Do it now."
The true value of a prop (screen used or replica) is how much you enjoy it. If you would enjoy owning this piece more if it were weathered, then do so. Whether or not the prop is licensed has no bearing on the situation as far as I am concerned.
That stated: It is much easier to be brave about modifying an inexpensive and/or readily available prop... not because there is less financial loss, but because it is easier to obtain a replacement to try again if your modifications do not go as planned. If you have a rare and/or expensive piece then you have to consider your skill level and the likehood that the finished product will be as you envisioned it. Even the Pros drop the ball in the endzone ever once in a while. Bondo and paint doesn't fix everything, but it sure does fix a lot.
Again, if your skillz are up to challenge and you will enjoy the piece more if it is weathered, I say "Do it." and don't think twice about it.