English to French Law/Patents Law: Contract(s) Motor Vehicle Transfer

If due to extraordinary circumstances the purchaser is unable to apply for a certificate of title within 30 days of the purchase date, the Director, in the Director's discretion and *for good cause shown*, may grant an extension of time within which the purchaser must do so.

My try:
Si en cas de circonstances extraordinaires l’acheteur se trouve dans l’impossibilité de faire sa demande de certificat de titre, le Directeur, à sa seule discrétion et *sous motif valable*, pourra accorder une prolongation.... (not quite sure how to finish sentence either.. at the moment !!)