That then begs the question, why does the cdi have a ground wire if the circut is completed through the spark plug and engine?
Wow, you really are an electrical novice.
It takes TWO METAL CONTACTS to complete ANY electrical systems parts.
Spark power is generate from the magneto in the motor through the CDI.
It has 2 contacts, spark voltage and ground.
The spark plug has 2 contacts, the high voltage spark at the top, and it's ground through the motor to the magneto.
In between is the Capacitor Discharge Ignition (CDI) module, the black box with the spark plug wire and TWO thin wires.
Remember, ALL electrical circuits need 2 connections, hence the black and blue wires from CDI, but can have more outputs.
What the CDI does is drastically increase the wimpy voltage for the magneto to a voltage high enough to arc across a spark plug gap and fire up your motor.
They ALL SHARE A COMMON GROUND FROM THE POWER SOURCE, HENCE THE MAGNETO GROUND!
The wimpy blue wire voltage in is boosted to like 1,000V and that goes out through the spark plug wire.
Heck, if your motor is not insulated from the bike frame with tire tubes or frame paint or something, you can attach the CDI black wire anywhere on the bike frame.
Do you know what a Muiltimeter is?
It is a $12 item at most any Radio Shack or hardware store, and it can measure Multiple types of electrical circuity, hence the name Multi-Meter.
Get one.
Set it to Ohms and use it's two leads to check if you have ground from the Magneto ground lug at the top to the head (for the spark plug) and the black CDI wire as well.
Opening up the magneto, tapping new grounds? Jezz kid, this isn't rocket science and about as easy as it gets.
My advice is just get the cheap tool and learn how to use it, you will find it will be a tool you use for way more things for years to come than your bike like for example seeing if there is power in a wall socket in the AC mode, or if a battery has any voltage in DC mode.
Ya falla? (I couldn't resist that line from the movie The Sting hehehe)