Removing unwanted color, make just b & w

[carljong]

Attached is an admittedly bad scan of sheet music.  It is not being recognized by my piano notation software.

I'm familiar with Colors > Curves to clean up sheet music, but it is not working on this photo.

I saw Despeckle, but the only effect it seems to have is to make things worse.

I wish I could just capture a sample of the bad section, say "this is the pattern", and get rid of it.

Thanks