Ok, they're working now. Here's what I did:
During the install, there was a separate "Decals" folder with some bort numbers in it. I simply added them to the \Objects\Decals folder. Some of the MiG-29C skins use these.
For skin-specific numbers located in "d" folders, I created an aircraft folder under \Objects\Decals, then a skin folder, then dropped the "d" folder into the skin folder.
So, for a path to the bort numbers for the MiG-29A of the 119th MIAP:
\Objects\Decals\MiG-29A\119\d
The 119's decals.ini needed a tweak; other skins' decals.ini files did not.
[Decal001]
MeshName=RightEngine
DecalLevel=2
DecalFacing=RIGHT
FilenameFormat=[b]Mig-29A/sov2/d/rbn[/b] <======change to Mig-29A/119/d/rbn
Position=0.100,-0.250
Rotation=0.0
Scale=1.3
DecalMaxLOD=3
[Decal002]
MeshName=LeftEngine
DecalLevel=2
DecalFacing=LEFT
FilenameFormat=[b]Mig-29A/sov2/d/rbn[/b] <======change to Mig-29A/119/d/rbn
Position=0.100,-0.250
Rotation=0.0
Scale=1.3
DecalMaxLOD=3
And now my MiGs are numbered.
Thank you both for your help.