I've been lucky that most of my locomotives just require a good cleaning to be able to run.
One of my basket cases was this prewar #231 which I bought on eBay from a guy who could not even tell which locomotive it was. I even told him where to look (for the number plate on the smoke box). I knew that the wiring looked to be shot. The seller then sold the tender (for a #230 locomotive) separately which I successfully bid on, too. Luckily, I have a friend who is the "McGuyver" of toy train repair and he was able to rewire the loco for me. Typical for these engines, the marker lights on the engine were gone. I replaced those along with fabricating a cab step from copper. The locomotive runs just as great today as it did when it came out of the factory 75-ish years ago.
Got any good stories about turning a "basket case" into a good running locomotive or fixing up a car, accessory, etc.?
Tom