I suggest the WEB, places like (see the link below)
There are 5 different models of the Desert Eagle BB gun not including the Baby Desert Eagle. Specify which model and Maker you are asking about. regular BB Gun or AirSoft.
I have never seen an all metal desert eagle bb gun, i have seen an all metal desert eagle .177 pellet gun from GAMO. I would assume if you go to GAMO.com you would be able to find it there.
The Magnum Research Baby Desert Eagle shoots Metal BB's. It is an Air gun not Airsoft. But Magnum Research also makes a Desert Eagle ( Not Baby) that is an airsoft pistol that shoots Plastic BB's
The Desert Eagle is distributed by UMAREXUSA. I would contact their Customer service and ask for a diagram if available. See the link below.
About 100yd
It's just there for looks.
it depends on if its in good shape or not
All of the Sears Model 799.***** BB guns were made by Daisy air gun .You may want to contact them directly or search the web for an 'Air gun repair" shop to locate someone to repair the BB gun for you.
i think any m4 mag can go in it
If its a BB Gun, Then Yes.
at big 5, a gun shop, Basspro, Cabelas or sportingoods store.
The reverse of how you took it apart. I would look on the web to try to find drawings of how to assemble the BB gun. Try looking at the company that made the BB gun first, some offer drawings of the gun and how to assemble it. www.Crosman.com offer this for most of their guns. OR you can take it to an Airgun repair shop to heve it re-assembled. Check "Airgun repair" on the Net and find a shop close to you.