I have fixed cracks like that before, in seatstays, very close to the dropout. The way I like to do it is to widen the crack and file the tubes so that they are square, then make a plug-type fitting that fits into the long portion of the seatstay, and has a tongue shaped end on the other side to engage with the dropout tab. I slot the droput tab and remaining tube still brazed to it, to receive the tongue, making a sort of tongue and groove joint between the plug and the dropout. This renders the remains of the tube that is still brazed into the dropout mostly cosmetic. I spring the seatstay away from the frame and silver braze the plug into the stay. Then I spring it back into place in the dropout/remaining tube and silver braze it into place there. It works. You can also braze it up all at once, but I prefer to do two operations.