I agree on the need for bushed firing pins. This might turn out in the long run to be an improvement.
Similarly, an entirely new set of springs and such might turn out positively. They'd be fresh and probably a bit less likely to break. Alternatively, since this gun does appear to have been made to a pretty-standard design, it may be that there are old parts guns lying around which could be cannibalized for their action innards. This is a question for a good gunsmith who handles these more or less regularly. If he can pop old parts into this gun and have it work, that would save days of time wielding a file and a lot of money.
I also agree on sleeving the rifle barrel. It would be a good idea, when considering this, to keep in mind that this gun was designed for low-pressure cartridges and to sleeve and chamber it for something mild. I'm thinking .30-30 Winchester, .30-40 Krag, .32-40, .38-55, .25-35 Win. or something along those lines. I don't know whether there's enough meat in the barrels to, say, open it out to something along the lines of .45-70, but that could be fun if it could be done. Any of the ones I listed would suffice for just about any European game. In any event, because of availability and cost I would want to have it in a readily-obtainable cartridge. The 9.3x72R gets pretty expensive when compared to the others.
My big question remains just how thoroughly the police messed up the shotgun barrels.
