It would depend on exactly where the splotch is. If it's on the outside of the viewfinder, you could simply clean it off with a damp cloth (and maybe some Windex or Formula 409). If the eyepiece is damaged, you can order a replacement part
here.
Remove the lens and look at the mirror. If their is something on the mirror, you can
carefully clean it. The mirror is very sensitive and vulnerable to scratching. If the reflective coating has been removed then you'll need a new mirror.
Look up at the bottom of the focusing screen. That's the more-or-less clear piece of glass that the mirror will reflect light from the lens onto. If there's a splotch on the outside, you can clean it off.
Those are the parts you can get to. If the splotch is on something on the inside, then you'll have to take the camera to a good camera shop to have a trained professional disassemble and clean it.
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