If you check this link http://www.rrpicturearchives.net/locolist.aspx?id=srs a lot of the US Sperry railcars started out ad Doodlebugs. Perhaps that might be a good starting point? Then it is a case of relocating doors and windows to match a specific car (see previous link) and then doing a paint job,
Walthers have done one in HO. I'm not sure what you call "large scale" I did find that MTH did a UP "inspection car" in O scale 3 rail that had central "rail testing" bogies fitted in about 2015 - see https://www.youtube.com/watch?v=9-0WrwTcWSY
Good Luck with your Doodlebug hunt,
Unanderra in oz
Looks like Aristocraft did a Doodlebug in about 2008, that might provide the best starting point, but my suspicion is that it will be a "buy and bash" rather than a straight "buy for display as purchased"