Despite its rarity, this Stanley hand plane is worn out and used up. Whoever owned and used it got their value from it. There is nothing left.
The soul of a hand plane is its sole. The rest are parts that attach. Those parts can be swapped around between planes, but the sole is the part that IS the hand plane. If you buy a replacement sole, you are buying another plane. That's okay, but you will have two planes, and will be restoring the second one, maybe with parts from the Stanley plane that you posted here.
I don't know much about metalworking. I do not know if it is possible to attach more metal and sand it down to flat. I have seen people do amazing things when they decide to do it, but my experience with hand planes tells me that this idea is a bad one. I don't think it would work, but I don't have enough experience to know it.
You said that you want to use the hand plane after you restore it. I worry that after all the hard work of restoration (even if it is possible?), the hand plane won't work well at all, and you may never know why. That's the honest truth.
If you are looking for a good hand plane to use for real woodworking, perhaps a different one would be a better choice, even if it is less collectable than the Stanley hand plane that you found. The shavings will never know which plane was used to cut them off. :-)
I never thought I would say this, but if you want that Stanley, give it a nice retirement: Buy it, clean it up, shine it up, wax it for protection, and then proudly display it on a shelf for decoration in your shop. Use a different hand plane for woodworking.