Gear box oil

hi there , does anyone know how to change the gear box oil on a Peugeot ludix blaster? I can see where to refill it but I can't find the drain plug ,

 

Drum brake it'll be just above/ behind the brake cable spring on the very bottom of the casing.

 

Thanks for that I been looking everywhere, all so think it's running a little rich cos plug is wet, how do you adjust the carb?its a standard gurtner carb with 60 jet standard bike but de restricted

 

What is the oil grade