Have an older gx620 in which started back firing and running rough then quit and won’t start. Put spark tester between plug wire and plug, no spark. Put tester between plug wire and ground and it shows spark. Have installed new coils, plugs, and cleaned magnet. Coils are gapped at .010” flywheel key is good. Grounding wires are disconnected from coils. I am at a loss and ready to replace engine but would rather not. Any help would be greatly appreciated.

First, get two new spark plugs.
 
With no installed spark plug, plug the spark plug wire lead into the plug and ground the plug to the head.  Crank over and check for spark (both sides) AT THE PLUG.  
 
.Post back what you find..

No spark