This set me on the right path, thanks!
import unreal
al = unreal.EditorAssetLibrary()
tas = al.load_asset("/Game/TestAsset")
tas.set_editor_property("complex_collision_mesh",tas)
bs = tas.get_editor_property('body_setup')
bs.set_editor_property('collision_trace_flag',unreal.CollisionTraceFlag.CTF_USE_COMPLEX_AS_SIMPLE)
al.save_loaded_asset(tas)