Arnold render settings are stored as a Video Post node. To set up the render settings programmatically you have to create a new video post instance and add it to the active render data.
The CINEMA 4D python SDK has a limitation and does not allow creating video post nodes. As a workaround C4DtoA offers a command to create the Arnold video post. The command id is 1039333.
Here's a simple example which returns the active Arnold render settings video post or creates and activates a new one if it does not exist yet:
If you have the video post data you can change any render settings as you would do normally in other nodes: