How to repair (not replace) cracked lintels/sills?

This is one of the windows on the front of our house. We have had some damp problems in that corner of the room and these cracks have been there a long while. I plan on removing the old peeling paint, repainting the masonry, and resealing the windows, but I wondered if there was a way of filling and sealing these cracks before painting them, to help prevent water getting in? Obviously I don't want to have to replace the sill/lintel if it's not absolutely necessary! Thank you in advance