How to add text lines to multiple files

Hi I have hundreds of cfg files that are used for configuring overlays in RetroArch emulator.

Each game has its own config as shown -

overlays = 1overlay0_overlay = 1941.pngoverlay0_full_screen = trueoverlay0_descs = 0

I simply need to add this to each file -

overlay1_overlay = blank.pngoverlay1_full_screen = trueoverlay1_descs = 0

The overlays = 1 at the top will need to change to 2 but I do know how to do that. Here is the end result needed.

overlays = 2overlay0_overlay = 1942.pngoverlay0_full_screen = trueoverlay0_descs = 0overlay1_overlay = blank.pngoverlay1_full_screen = trueoverlay1_descs = 0