you have to set the clock as 50Hz by manipulating the %)/60Hz signal
But, bear it in mind that, the public mains in countries like India is never 50Hz. it is near 48 or at times 47.xx with somuch of national GRID. Thus your clock will never show correct time.So the need for crystal time base.
Once crytal time base iss used for accuracy, the display can't perhas be driven.
this is like a "seed or tree -first" situation. i hope you appreciate that both problems land in the same solution. OK, anf size purpose, you may use SMD components and tuccck the add on to the main clock. Yes. it can run from mains and display can be blanked during failure, but when mains restore the display will be driven by the crystal derived signals only and not from mains as the lamps start blinking and will be blurr.