Small Engines (Lawn Mowers, etc.): Riding mower stopping running, briggs and stratton, safety switches

Question
I have an old Montgomery Ward 42" Lawn tractor that just stopped running in the middle of mowing the lawn.  I have checked all safety shut off's and for what ever reason the spark plugs are not getting any spark.
The engine is a twin 18HP Briggs and Stratton.  As far as I know only the Magneto gives spark to the plugs.  But this thing also has an Alternator.
As long as all safety switches are in the right posintion it will crank but not start.  which is how it is suposed to work.  Saftey switches also shut down mower if you do something you are not suposed to while mower is in motion.  what I need to know is should I start by replacing the magneto or go deeper into the engine and look at the alternator.  I don't know what else would be keeping it from getting a spark?
Engine model is 422707
Type is 1515
Go ahead and post this if it will help anyone.

Answer
Hi Dan,

Sorry for my delay.  The holiday was busy here.

I would suspect the coil/magneto.  If the wiring to it is correct and intact but still no spark, replace it.

Good Luck,
Michael

I am swamped with questions and won't be able to take follow up questions until next week.  Sorry.