Did the gunsight turn completely white? (Also, are you having view and sound bugs? ) If so, update your virus scanner and check for a virus. MANY people have been catching this e-mail virus called W32.hybris.gen. It causes this.
As for gunsights in general, normally all you have to do is make a gunsight you like and just name it default.bmp and place it in your sights directory. (Rename the old default.bmp to something like defaulto.bmp in case you want to go back.) This will make all fighters have the same gunsight. I believe if you name it spit9.bmp that would be for just the spitfire 9.
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bloom25
THUNDERBIRDS