Small Engines (Lawn Mowers, etc.): Stihl Backpack Blower BR400, stihl wont start

Question
My Stihl BR400 backpack blower won't start.  I replaced the spark plug with a NGK BM6A spark plug (I tried 2 new ones), but that did not correct the problem.  I replaced the oil and gas. I grounded the spark plug to the head and observed that I had spark at the spark plug.  I replaced the carburetor.  I checked the cutoff switch by removing the spark plug and grounding it to the cylinder head.  I had good spark with the switch in the run position.  In the "off" position I received no spark.  I assume the switch is working properly.  I also removed the fuel line and fuel filter to make sure it was clear of any obstructions or dirt, and found nothing.  I also removed the exhaust screen and found it to be clean.  The blower still won't start. How do I know if I have a bad magneto?  Also, if I have a bad magneto how do I replace it?  Do I need any special tools?
Thank you, I greatly appreciate your help.

Answer
You need to check the compression next.  It will take a compression gauge to do this.  It's not uncommon for a ring to stick in a 2-stroke.  Since the plug is firing, I wouldn't worry much about the ignition until after checking the compression.  Let me know what you get and we can go from there.  Thanks, PK.