Check the coil lead for cracks in the insulation--Especially near sheet metal or engine parts. Also check the spark plug connector. I believe they screw onto the coil lead. You can sometimes clip about 3/8"-1/2" off the coil lead and reinstall the spark plug connector.
If that fails to restore a good spark, you may be headed toward a serious expense. A new coil or ignitor as Kubota calls it will push 5 C-notes with tax and shipping. That's item 50 in the diagram below.
Edit: Also check for rust on the coil laminations. Too much can cause the coil to contact the magnets in the flywheel. The spacing between the coil laminations and the flywheel is critical for a good spark.