As I had actually made the Bf-109 ones back in the days of SF1, I could maybe add a few details.
Like already mentioned, the kill marks are defined by "DecalLevel=3". And like every other decal you can find them where "FilenameFormat=[some folder]/[base filename]" points to. In SF2 that would be in "Objects\Decals\[some folder]".
What i called [base filename] is the name after the last "/".
The files that are used fo the killmarks now all have to be in the format [base filename][number], with [number] being the amount of kills. So as a result, if you don't like how the killmarks are structured, you can create your own ones, one for ervery possible amount of kills you want to have represented (can be a lot of work ).
To make it a little bit easier to understand, let's take the current use of my killmarks as an example (yes, they are still used in current mods, albeit uncredited...):
Exempt from Decals.ini
[Decal1011]
MeshName=Rudder
DecalLevel=3
DecalFacing=Left
FilenameFormat=Bf-109/KM/Bars
Position=-5.78,0.82
Scale=0.65
DecalMaxLOD=4
Which means that the killmarks are in "Objects\Decals\Bf-109\KM\", named "Bars000.tga" to "Bars350.tga". In addition you will find a "Bars.tga", which if I remember correctly is used for kills > 350 (or to be precise, for kills not covered by the existing files), but it was so long ago I might be mistaken on that...
I hope this helps a bit, happy modding!
(And yes, I AM still alive, and reading on this forum. I just don't write much anymore )