Indian Railways are in the process of scrapping their YDM-4 and 4A fleet,
https://en.wikipedia.org/wiki/YDM-4" onclick="window.open(this.href);return false;
The the above statement "never was that common of an engine since only a couple of models of low-production switchers and light road units ever had them", appears to be North American centric. The 251-D was common in exports, still a few running in Australia, may be a place to look for core
DLW Varanasi built over 800 YDM-4 and similar export models for several African and Asian countries. The 251-D was used in other DLW built locos including the
WDS-5 and WDS-6,
http://www.irfca.org/faq/faq-loco2d.html" onclick="window.open(this.href);return false;
Parts still manufactured in India, the export agent is RITES
https://ritesltd.com/" onclick="window.open(this.href);return false;
Indian Railways/DLW/RITES are all parastatals/quangos, so you would be dealing with the most intractable of the human specie, the Indian Civil Servant
DLW built well over 1000 251-D. They purchased the rights, so have developed this engine independent of FM
Fepasa in Chile have a couple of Dt-12100 (DL-535 / RSD-30) stored at Alameda, at least one still has an engine, but if Chile is on the radar be quick as a local Facebook foamers group is saying that they are due for the gasaxe
Silvio Casagrande would be a good starting point for Chile, he has a former Dt-12100 in his loco hire fleet, and is still rebuilding 251-D, possibly using Indian parts, Refer División Ferroviaria at,
http://www.casagrandemotori.cl" onclick="window.open(this.href);return false;