Testing the spark ignition:
Remove the spark plug and connect it to the spark plug cap. Manually hold the spark plug with cloth onto the cylinder head (metal to metal) and make sure the side electrode is sideways so you can see the spark gap clearly. Push the bike to see if any spark is generated. It is hard to see in full daylight so it's best to do at night or inside. If no spark happens then there is a problem. If you have spark but the engine won't start then one of these problems exist:
1. Compression is being lost due to loose head nuts or non-parallel head/cylinder mating surfaces (that you can fix with sandpaper and glass).
2. There is an input air leak due to loose intake manifold connections or poor sealing where the carburetor connects to the intake manifold.
3. Crankcase compression is being lost due to bad crankshaft seals.
4. Something is really wrong with the carburetor such as a jet being clogged.
5. Someone (probably you) put the magneto on backwards so that the spark is happening at the wrong time.
Test the Stator Coil:
Use an AC voltmeter to test for 25-50 volts coming out of the stator coil as you push the bike with the spark plug removed. First disconnect it from the CDI. I prefer using an analog voltmeter such as this one: [voltmeter] If the voltage is there then the stator coil is OK although I would still remove the stator coils mounting screw that fastens the ground wire to the metal ground so that you can clean it for good conduction. Good metal to metal contact there is essential for the ignition system because that is the return path of the electric current creating the spark. A loose or rusty connection there creates resistance which lessens spark strength.
Test the CDI:
You can test for total circuit performance (without determining the peak output voltage which requires a special test circuit or a meter that can read peak voltage) by just holding the CDI output wires while pushing the bike. Someone may have to help you with that. Start out very slow and gradually push faster till you feel a minor shock. I prefer this basic method because the output voltage lasts for only a few thousandths of a second which most meters can't register. If you are getting shocked then suspect the high voltage coil or the spark plug cap or the spark plug as bad.
You can't test for resistance at the input or output of the CDI because the input is a combo of resistance and a diode, and the output is capacitance and a diode.
What normally goes bad in a CDI is one of the diodes, the SCR (black transistor with 3 legs), and the large output capacitor (the largest component on the board). Unfortunately the circuit components are buried in epoxy and impossible to get to.