I'd buy some bronze wool (available at most hardware stores), some Kroil or other penetrating oil, and bring some elbow grease along, too. You can get rid of the active rust that way and not lose anything worth keeping: bluing, browning, plating, patina. "Sentimental value" is a REAL value, IMO.
There are guys here who know Manufrance backwards, forwards, and sideways. I'm not one of them, but maybe one will chime in and let you know where this gun fits in the Manufrance world. I fairly recently visited their museum, but it is more focused on bikes and bicycles than guns, although there were a couple of drool-making "Ideals" there.
