Without seeing the gun it is impossible to determine how bad the corrosion is. If it is only at the muzzle the barrel can be cut down an inch and re-crowned. Even if the corrosion can be removed without cutting the barrel you would want to cut a new crown.
With a magnum caliber you want to keep the barrel at least 24" long to get the full performance out it. If the barrel is any shorter you will not get a complete powder burn while the bullet is in the barrel. It will decrease the velocity and cause a muzzle flash.
If the barrel is damaged beyond repair the cost of replacing it with a good quality barrel like Shilen or Douglas can be $500. The Parker Hale receivers are commercial Mauser receivers which are excellent for building a rifle on. There are a few pre-threaded and short chambered barrels available, but very few are in a magnum caliber.
I hope this helps you, if you need any other info don't hesitate to ask.