First of all you never and I repeat never ever store a gun for any length of time in a gun case. You have seen the result of that, it is almost a guarantee the gun will rust. Next the best thing would be to have the gun and hardware re-blued by a professional. If you want to do it yourself I recommend you remove all the old bluing. If your barrel and hardware is pitted you should do you best to remove the pits and polish the bare steel. I sporterized an 8mm Mauser a long time ago for my brother. I won't go into all the details of fitting and finishing a new stock but I will tell you how I reblued the rifle with cold blueing, Birchwood Casey. After the barreled action was stripped of old blueing and polished I got a long tin chicken feeder trough about 4 inches deep and long enough to hold the barreled action. I plugged the muzzle and the Breech with pieces of wooden dowel rods and put water in the trough and set it on my stove top and brought it to the boil. When it was hot enough to flash dry I took it out of the water and using a and continued the process until the barrel was deep black and shiney, about 20 times. That was back in 1970 and the barreled action looks as good today as it did in 1970. Here are some pictures. It can be done, it just takes time.