That is a good clean break, so should be a fairly easy fix.
You will ned to purchase some Brownells Acraglass Resin, NOT Gel , should cost under $30.00. It is a 2 part epoxy made for gun work. Take the broken off part and drill a few 1/8" holes into but NOT through the cap, do the same thing in as close to the same spots in the stock, you can use a bigger bit to help ensure getting hole alignment.
Mix up a little bit of acraglass, put a drop or 2 into each hole of the cap and stand a short metal pin, like cut off paperclip in each hole. The paperclip pieces should be about 1/2" long, then let the glass cure for a day, when the pins are hardened in place try to drop the cap onto the stock, open up the holes a bit if needed in the stock till the cap drops right into place. Then mix up more glass fill the holes in the stock , put a light coating of glass on both exposed wood surfaces then stick them together, place the stock upside down , make sure the cap is on in the RIGHT alignment and let is cure for a few hours, some of the glass will ooze out, but as long as it is still soft will clean off with vinegar.
Once the acraglass sets up the stock will be stronger at the break point that it was to begin with, if done right you will not see the repair.
Good Luck
:mrgreen: