From what I can see it looks pretty original which is always nice. Unfortunately it looks like someone has been holding down barbwire fence with the cheek of the stock.
Damage like that is hard to repair.
Like David says, restoration cost would exceed the value of the gun, however a cleaning using proper techniques could certainly make it look a lot nicer without robbing it of it's originality.
Using the proper techniques is key. Application of anything improper to a surface runs the risk of devaluing the originality.