One can do it yourself, but you have to find a spring first and then to adjust them in all cases. It's not a problem.
I was interested to look into this Boxlock. All enthusiastic double rifle big game hunter should look inside of his rifle and than maybe more than one would consider whether he want to continue to stand in front of a dangerous game with such rifle or rather use a classic bolt action rifle, built with a Mauser action for example. Apart from the fact that Boxlock actions and also Sidelock actions are not comparable to bolt actions that have proven themselves in various wars, the types of the springs inside, and there are more than one in this actions, are at risk of breaking in the long term. It may be that at the time when hunters often used double rifles for big game hunting there were hardly any problems because the rifles were not very old, but this broken spring made me think about whether one should nowadays still hunt such game with used old double rifles. I was never a big supporter of double rifles for hunting big game anyway.