Nice rifle Loki610.
I want one of these.
It is an easy job to stabilize the crack in the butt stock.
I did this with one of my MK IV martini Henry`s a few years ago.
Remove the butt stock and drill a small hole or two,(3/16" or less) from the face that fits against the receiver, towards the butt, inline with the crack.
I used a quick grip to hold the crack together while I drilled to keep it from splitting any more.
Wrap the stock in news paper and masking tape to protect it from your sticky fingers. Now force one of the many bonding agents used for wood down the hole with a toothpick, piece of wire or similar.
I use a bedding compound like Acra Glas or Bedrock, if I want to make it super strong.
If you can gently open the crack a bit while doing this it will force some into the split.
Clamp together with a padded C clamp or Quick Grip and let `re set up.
I would also make some shims the same size as the face of the butt, to put between the butt and receiver.
There should be a small gap all the way around the butt where it meets the receiver.
This will stop the this area from chipping out when you shoot.
Save your drill shavings to mix with carpenters glue to help hide repairs if you ever need in the future.
Put it back together and blast away.
Hope this helps.