I don't know of anyone who offers restoration, but it's at least possible a company that originally made the knives might be willing to do it.
Camillus is out of business, but Ka-Bar is still making the WWII type Ka-Bar Navy and Marine knife.
To rebuild, you need to make or buy a new set of leather washers.
Again, an original maker like Ka-Bar might sell new washers, or you can buy them from Texas knife maker's Supply.
You can make your own from 12 to 13 ounce saddle leather.
Many knives also had fiber spacers of red or black fiber material.
Knife maker supply houses will sell this in small sheets, and MAY have a source for pre-cut leather washers.
http://www.texasknife.com/ (Sell leather washers as "leather Ring Spacers").
You'll have to remove the butt cap from the knife, most of these are pinned and/or staked on. Some early WWII Ka-Bar type knives had the cap threaded on, but most were pinned and staked.
With the cap off and the old washers off, clean up the tang of any rust and inspect the guard for corrosion.
Stack new washers on, using the same type of fiber washers if needed.
You'll have to make some sort of compression press to compress the washers TIGHT in a stack while you re-pin or stake the butt cap.
You cannot compress the washer stack enough by hand, you'll need a vise or a press to do this.
Once the cap is back on, use files and sandpaper to shape the washers into the original oval cross section.
Use a round wood file to cut the grooves in the handle to match the original. Note that each maker had a specific spacing, width, and depth of grooves.
Once the handle is fully shaped, use finer and finer sand paper to smooth the handle, then finish with a couple of coats of Johnson's Paste Wax, or even a coat or two of fast drying polyurethane.
The only difficult part of it is compressing the washer stack.