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