If it is just a fine crack, scrap out a wedge and refill with putty. Do remember putty on the bottom & seal between the paint and glass matters most, the rest can be cracked and falling off. If a gap opens between the putty & glass it will serve to retain glass borne water and very rapidly rot through. Putty is quite tolerant of cracks if painted with a good alkyd undercoat & gloss, it fills them quite well.
Never apply a filler without first applying wood hardener. The wood hardener stabilises the wood from shrinking away from the filler and gives it something to bond to. Just check the next year nothing has opened, you can repair very severe rot in foggy saturated November with wood hardener but the following spring it can open a little around the filler, pipette in more wood hardener and all is happy thereafter.
Ensure the paint used is compatible with linseed oil putty, Trade Weathershield is however overcoatable paints like Sikkens Rubbol Undercoat & Rubbol do not adhere well to it (chips off or even falls off). Frame sealers can be more successful, but check for compatibility. The downside of Weathershield is it retains water underneath in its final year(s), I have watched relatively high moisture content pine rot in a couple of years with Weathershield as it forms a plastic bag of water, whereas the alternate timber above painted in sikkens Rubbol Undercoat & Rubbol actually went to low moisture content over the same period as it could breathe much better.
Very nice window and the brick arch & pointing are in great condition.