how to clean dirt/yellowing from old document

[gimptime]

I am working on an old baptismal certificate. It has a white background. There is what looks like
dirt on some of the background, which I would like to remove. It is also yellowed to varying degrees
which I would like to fix. I have used the brush/pencil tool and painted some of those areas
with a lighter color from the background but its too slow. Any ideas would be greatly appreciated.

Thanks.