the answer can be found in the OCIO examples, specifically here. In the example I do the exact opposite from what you want to do, I convert a scene from legacy/basic mode to OCIO. Its inverse would look somewhat like what I show below. I hope this helps.

Cheers,
Ferdinand

"""Demonstrates how to convert a document from OCIO color management to basic color management.
"""
import c4d


op: c4d.BaseObject | None # The primary selected object in the scene, can be None.
doc: c4d.documents.BaseDocument # The currently active document.

def main() -> None:
    """Runs the example.
    """
    # The document is already in basic mode, so no conversion is needed.
    if doc[c4d.DOCUMENT_COLOR_MANAGEMENT] == c4d.DOCUMENT_COLOR_MANAGEMENT_BASIC:
        return c4d.gui.MessageDialog("The document is already in basic mode. No conversion is needed.")

    # Get the converter, and the active render space, which by default will be ACEScg.
    converter: c4d.modules.render.SceneColorConverter = c4d.modules.render.SceneColorConverter()
    renderSpace: str = doc.GetOcioRenderingColorSpaceNames()[0]

    # Initialize the converter with:
    #              doc, from-low   , from-high   , to
    converter.Init(doc, renderSpace, renderSpace, "scene-linear Rec.709-sRGB")

    # Convert everything in the document (we pass the document itself as the second argument). The
    # undo management is only needed so that our DOCUMENT_COLOR_MANAGEMENT below is reversible. The
    # ConvertObject call creates undo steps on its own with the default flag we pass. 
    doc.StartUndo()
    doc.AddUndo(c4d.UNDOTYPE_PRIVATE_DOCUMENTDATA, doc)
    if not converter.ConvertObject(doc, doc):
        print(f"Failed to color convert document '{doc}'.")

    # Finally, set the color management mode to basic/legacy. It is absolutely important that we do
    # things in this order, otherwise SceneColorConverter will not work correctly.
    doc[c4d.DOCUMENT_COLOR_MANAGEMENT] = c4d.DOCUMENT_COLOR_MANAGEMENT_BASIC
    doc.EndUndo()
    c4d.EventAdd()
    
if __name__ == "__main__":
    main()