It can be replaced, but is it worth it? I've had mirrors fixed/replaced a few times, and it's cost $175 and $225. Depending on the model it may be cheaper to replace the camera.
On the other hand a scratched mirror is a purely cosmetic issue. It may bug you looking through the viewfinder, but it won't affect the photos.
Matt, I see your point. Well it was more on wanting to learn film photography and since I had the tools why not use it. I just thought that the scratched mirror will be an issue, but since it won't affect the image, then I'll just put up with it.
Learn away! The mirror is just there so we can see through the viewfinder what the lens is seeing. That fast clunk-clunk noise when you press the shutter button is the mirror swinging out of place while the photo is taken, and then dropping back into place. If you look through the viewfinder as the photo is being taken with a SLR you'll see that it blacks out for a split second. That's because the mirror is no longer in place.
The scratch itself is just annoying. Now hopefully whatever scratched the mirror didn't do any other damage, like mess up it's alignment. Shoot a test roll to make sure everything is working, and then learn away!