how do i fix 3.5hp briggs&stratton mower engine?

No spark? Replace coil. You might check for continuity from HT wire to ground, and from kill wire to ground. Is it old enough that it has points and condenser? I am experienced with pre '70's engines. The Magna fire (or whatever) is new to me. I just discovered that while working on some '80's and later engines. They have no breaker points. Someone told me that there is an "exciter" built into the coil that provides spark at the right time. Old coils just quit for some reason--even automobile coils. Sometimes trying to restore an old engine is futile unless you have the time and patience to tinker with it. You will learn what WON'T work. Good luck

Robert Brown
August 2008