how do i fix the sparking unit behind the fly wheel?

hello i have just been given a very old briggs and stratton engin i am half way through reparing it and have found that the small black button that is part of tha sparking system that moves the piece of metal up and down on to the condensre to make the spark isnt moving . how do i fix this?

a am also sory to say i cant find the engin no or serial no i have looked in all the uasil places but haid no luck

thanks