Your brother has found a nice Airstream Liner. For reference, look at R.J. Dial's web site for photos of other Liners, both originals and modifieds:
Vintage Airstream Photo Archives
The side windows will be difficult to find. Someone rounded the square outer corners to reduce injuries when walking into an open window. The original square corners are sharp! Your best bet for finding matching windows is an old Airstream or Curtis Wright body shell being scrapped out in a rv salvage yard, or buying windows from a vintage trailer restorer who is willing to let some go. Finding a body shell from 1947 to '49 will be challenging, so begin thinking of alternatives windows in the same style.
The side marker lights were glass lens beehive types, red rear and amber front. Prowl old car/old truck swap meets for glass lens and metal housings. You might use some new plastic potted led beehive marker lights until you gather a full set of original ones.
The original butane tank was likely a small horizontal one. Since you already have the rack for them, from a practical standpoint your brother will be better off with two small vertical ones having modern OPD valves similar to those now on there.
Move the beautiful deco AIRSTREAM nameplate from above the front window to below the rear window, where it belongs. It holes may still be there. Have fun!