Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Text Spline
PRIM_TEXT_TEXTParameter: Font
PRIM_TEXT_FONTParameter: Align
PRIM_TEXT_ALIGNParameter: Height
PRIM_TEXT_HEIGHTParameter: Horizontal Spacing
PRIM_TEXT_HSPACINGParameter: Vertical Spacing
PRIM_TEXT_VSPACINGParameter: Separate Letters
PRIM_TEXT_SEPARATEParameter: Kerning
PRIM_TEXT_KERNING