I am using v5.22DEV x64
Editing a MAME rom to use Libretro/MAME core has no effect. The rom is still launched using mame2003.
Update: If I set the default emulator for MAME (game settings --> advanced --> MAME) to Libretro/MAME, it uses the proper core.
Did the Mame Rom's are now playable? I use Mame Emulators to Play games on my PC.
Now I'm playing Metal Slug X, and with the emulator, the Mame Rom's ( https://www.retrostic.com/roms/mame ) are perfect to play like the old days.