First of all, are you sure it's really gone? The place that item normally lives is here:
Macintosh HD/Library/Application Support/Logic/Channel Strip Settings/Track/03 Acoustic Guitar/Acoustic Becomes Electric.cst
When you browse in the Finder to that exact location, are you sure it's not there?
If you had used that setting and saved it as your own (probably after modifying it), then it would normally end up here:
Macintosh HD/Users/YourUserName/Library/Application Support/Logic/Channel Strip Settings/Track/Acoustic Becomes Electric.cst
That's another place you might want to look. Or maybe it's possible that you deleted it from the latter location, and don't realize that there's still another copy in the former location.
You could also try looking for it with Spotlight. But if you do that, you need to tell Spotlight that you are interested in seeing System files. More details about that here
If the item is really nowhere to be found on your drive, you can recover if from your Logic install disks. The easy way to do this is with Pacifist
. Insert your Logic Studio Install disk. Launch Pacifist. Press the button "Open Apple Install Discs" and then select "Logic Studio (Logic Studio Install)." It will take a little while to open. When prompted for the other disks, just say "Skip."
In the window that appears, enter 'becomes' into the Find field and press return. You will see the item you're looking for. Now just drag it into the proper folder, as I noted above.
This is much faster than using the regular Logic installer to get the item, because the installer won't let you pick out exactly the item you need, the way Pacifist does.
I keep my Logic disks stored as disk images on an external drive. That makes this process much easier, and I can keep the physical disks stored in a safe place where I never touch them. So the risk of them being lost or damaged is greatly reduced.
I may have deleted others whilst starting to use the program, and wonder if there is a way to restore all from my Upgrade CDs?
Everything I said above should also apply to the other items you're talking about.