A quality driver should help, My father in law works a lot with car mechanichs and he has a set of Snap-on drivers to use when others don´t work.
Also, when I get a new harmonica I dissamble it to change the tunning in a couple of reeds, the screws in Hohner harmonicas are Reed and Prince and finding the right driver in México is not easy so I have cut slots with a dremel on those small screws more than a few times and it allways worked fine, just have to be carefull not to cut yourself or damage anything.
If all else fails you can get a screw extractor, you drill a hole on the screw and insert a conical bit that gets tight as it is inserted while twisting, it grabs on and takes it out, it may work in the existing hole of the torx head.