Fox Collectors

I'm having a heck of a time trying to remove this screw on my Sterlingworth. It won't budge and I don't want to bugger it. I have tried lightly tapping on the screw driver before torking it. The stock is broke. A corner piece broke off on the upper right hand side near the receiver. I can see all the way through to the firing pin hole. I would like to remove the stock for repair or replacement. Any suggestions?

Thanks.