OUCH! Well I've seen that type of crack before..., it was in a Pedersoli Bess, and it was fixed once before I got to it, so I was fixing the second crack. I figured out that whoever drilled the circular hole for the sear lever, drilled it a tad too deep. So I cut off the tip of a dowel that fit the hole, just a small disk of hardwood to reenforce the repair.
Now I took a 3/32 drill bit, and drilled several holes in the wood perpendicular to the crack, from the top down. Then I used Brownell's Bedding Compound and 1/8 wood doweling in the holes, and a couple of clamps, to set the repair. I installed the disk I mentioned above, which was about 1/8" thick, and used the bedding compound on it as well. I had to shorten the seer lever tip just a bit, but it still covered the internal trigger slot completely..., and the repair has held ever since (2008).
Now in your case you might want to allow the compound a dowels to stick up just a bit, and then sand them flush. You probably will need to refinish the stock a darker color, to cover the repair or make it less obvious. IF you can open the crack a bit, you might put some bedding compound in there too, but that's risky for if you put too much pressure..., you may break the stock apart.
FYI it's only a wild arsed guess that the hole was too deep thus weakening the stock a bit too much..., but the repair has held, and couldn't hurt as long as you don't change the interior area so much as to mess with the moving parts of the lock.
LD