CDI unit? That is a fancy technical term for ignition coil. A simple test to check for ignition fire is to remove the plug and ground the metal end against the block, then have a helper slowly pull the start rope while you watch for spark. If you have a bright blue spark, the coil is working. An adjustable spark tester also works very well. The flash bulb type ignition testers are a gimmick and unreliable. Was to coil installed upside down? It is a simple mistake that will still show spark, but it is literally out of time with the piston and valve operations. Also, aftermarket coils have been known to be internally wound backwards before and that will keep a person guessing. Was anything struck on the ground when the mower suddenly stopped? If so, the flywheel key could be sheared and that will throw off spark timing. Check this parts link out:
Parts lookup and repair parts diagrams for outdoor equipment like Toro mowers, Cub Cadet tractors, Husqvarna chainsaws, Echo trimmers, Briggs engines, etc.
www.partstree.com