FreePlay Zero Remap Controls

[Sithel]

Since I built my FPZ with no X and Y buttons how do I remap those on a per emulator? For example, seems like Mega Drive uses one of these when sega just uses A,B,C , I could remap C to left shoulder or right shoulder button. I tried coping GPIO controller config, just hte left shoulder input_l_btn = "2" to the retroarch.cfg in megadrive , but it doesn't work. Any help would be appreciated.

Thanks