Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Count
MG_RADIAL_COUNTParameter: Radius
MG_RADIAL_RADIUSParameter: Plane
MG_RADIAL_AXISParameter: Align
MG_RADIAL_ORIENTParameter: Start Angle
MG_RADIAL_STARTANGLEParameter: End Angle
MG_RADIAL_ENDANGLEParameter: Offset
MG_RADIAL_OFFSETParameter: Offset Variation
MG_RADIAL_RANDOMParameter: Offset Seed
MG_RADIAL_SEED