Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Velocity Stretch
MG_PARTICLE_STRETCHParameter: Particle Scale
MG_PARTICLE_SCALEParameter: Map Animation to
MG_PARTICLE_MAPPINGParameter: Distance per Second
MG_PARTICLE_DISTANCE_SCALE