A building or structure devoted to a nonconforming use (whether in whole or in part) and a building or structure nonconforming as to height, setback or yards may, if damaged or destroyed by fire or other accidental cause, be repaired or reconstructed within the same portion of the lot, to the same or lesser dimensions, and used as before, without a finding as provided in either Subsection A or B, provided that such repair or reconstruction is substantially completed within two years of said damage or destruction; otherwise, such building shall be repaired, reconstructed and used only in conformity with the provisions of this bylaw. The Board of Appeals may, however, in a specific case, authorize the extension of the two-year time limit or authorize a special exception from the requirement of conformity, where, in the opinion of the Board of Appeals, such limit or requirement will impose undue hardship.