How to open a Casio FX-4000P?

[johanw]

I got a 2nd hand Casio FX-4000P and the only reason it is not working is that AFAIK the batteries are empty. However, the 2 screws on the back have apparently been opened and fastened by someone with either the wrong tool and / or no feeling for it with much too large a force so the heads are damaged and I can't unscrew them normally.

The head is very shallow, with small Torx in an old Nokia phone suffering from the same issue I got them loose by soldering a screwdriver in the screws but I don't think that is going to work here.

Any advice? Is there a way to get those screws off without damaging the rest of the calculator? And where would I look (from Europe) for decent replacement screws?