** I overcooked a bag of popcorn. Yes the smell is evil!! **
It is sometimes possible to reduce the smell by repeatedly boiling a large container of water with a bit of lemon juice in it.
BTW. Don't try to move or stir boiling water after being microwaved. It could explode leading to serious scalding!
Just boil it, let it cool then boil it again, etc.
** but the back of the microwave has a large yellow stain **
Microwave cavity touch-up paint is available in small spray cans (see the following link) which can be used to cover any imperfections... after preparing the surface (just like any other spray paint).