Hello @kazokutchi
From below comment I assume M5Stack engineers decided to not include the SetChargerCurrent() function for M5StickCPlus.
// Not recommend to set charge current > 100mA, since Battery is only 80mAh.
// more then 1C charge-rate may shorten battery life-span.
That said, have you tired to only copy the function by itself into the corresponding AXP192.cpp file of the M5StickCPlus library?
Thanks
Felix