All except one were regauged. All except one are still operational.
The original MAK loco was regauged from its original 860mm to 1000mm for use on the Brohltalbahn and RhB. It still runs on the RhB.
V51 901 was regauged from 750mm to 760mm for use on the StLB in Austria then regauged back to 750mm for use on the RuKB.
V51 902 has stayed on one of its own original lines since being built and so has always been 750mm.
V51 903 was regauged with swap bogies for maintenance work in Southern Spain (1000mm narrow gauge / 1435mm standard gauge / 1668mm Spanish broad gauge). It has since been repatriated with V51 902 on the Oschle Museumbahn and will be put back to its original 750mm.
V52 901 and V52 902 were both regauged to 1435mm for use on local private lines to recoup the local grant that was issued to build the locomotives which were deemed not to have repayed their purchase price for the region. V51 901 had a larger cab fitted to make it more standard gauge sized but looked hideous
http://www.loks-aus-kiel.de/images/basic/gmdr_5325_51.jpg . It eventually found itself in the hands of a private maintenance company in Italy. V51 902 was not so lucky. Despite keeping its original profile and eventually being regauged back to 1000mm for use on the Italian FGC narrow gauge route, it was later to be rebuilt as a self propelled snow plough, but this came to nothing and the loco was apparently cut up for scrap.