You'll need to contact theymos as he's the only one who can do that.
How can he do that? I don't think there is a chance unless he uses some kind of SMF Mod to store deleted messages. The only thing that they can do is if there is a database backup that before the OP deleted the message from his inbox then there might be a small chance but i don't think there is and i don't think that no one would bother themself with this much work.
Direct quote from simplemachines.org
PMs exist only in the sent box of the sender (if enabled) and the inbox of the receiver. Once deleted from both they are gone forever.
Even if they weren't fully deleted, violating members' privacy is not considered acceptable and even if that is not your intention, it is policy not to provide any instructions that could help others to do so.
So OP if it's possible you should ask the receiver.