Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Strength
MDATA_FITCIRCLE_WEIGHTParameter: Roundness
MDATA_FITCIRCLE_ROUNDNESSParameter: Exact Radius
MDATA_FITCIRCLE_ABSOLUTERADIUSParameter: Scale
MDATA_FITCIRCLE_SCALEParameter: Radius
MDATA_FITCIRCLE_RADIUSParameter: Angle
MDATA_FITCIRCLE_ANGLEParameter: Adjust Inner Points
MDATA_FITCIRCLE_ADJUSTINNERPOINTSParameter: Project To Surface
MDATA_FITCIRCLE_PROJECTTOSURFACEParameter: Equal Spacing
MDATA_FITCIRCLE_PROJECTREGULARParameter: Select Outline
MDATA_FITCIRCLE_SELECTOUTLINE