Since money is an issue, just swap out the magneto coil and CDI with your friends to see what works and what doesn't with yours.
If you could borrow a volt meter from a neighbor, you could easily find out if it is the magneto coil or CDI:
Disconnect the CDI from the magneto coil. Connect an AC voltmeter to the magneto coil wires (blue and black). Push the bike with the spark plug removed and see how much voltage is generated. Up to 50 volts is good. Drastically less indicates a bad magneto coil. If the voltage is sufficient then it's probably the CDI.
As a reminder, if you left the bike out in the rain or rode it in the rain, the magneto coil may have become corroded.
A spark plug wire disconnecting while the engine is running may also damage the CDI. It's also important to ground the plug against the head when checking for spark.
Good Luck,
Chris
AKA: BigBlue