Try the simple things first, like change your spark plug. Lay the plug against the head and see what color the spark is, or if it's present. Blue/white is good, orange or yellow is crap. Then check to see if the boot or the wire leading to the spark coil has come loose. Check to see if the PEI connector is loose. Clean the contacts.
If it's still not working, test to see if the ignition is grounding out. Disconnect the PEI, turn the key and On switch on. Find the black w/red trace wire pin. See if there is continuity between that pin and the frame. If there is, the ignition is grounded somewhere and that's bad. If you find a fault, then you will systematically have to go from connector to connector, checking each wire to isolate exactly where it is occurring.
Isolate the two sides of the magneto coil. Look further down the loom from the PEI and locate the connectors for the black w/red trace & the red w/black trace. Disconnect them. Continuity between them is good, that means the coil isn't grounded out. If either connector has flow to the frame then the fault lies somewhere in those wires or at the coil itself. Leave these wires disconnected for the rest of the test.
Find the black w/red trace connector at the on/off switch. Disconnect it. Find the black w/red trace connector at the key switch. Disconnect it as well. Check from the black w/red trace pin on the PEI connector to the two switch connectors.
Test the on/off switch itself between it's wire and the handlebar. Clean the switch's internal contacts. You should only get continuity when the switch is off.
At the key switch check between the black w/red trace & the black w/white trace connectors. When the key is Off you should have continuity. In the On position, you will not.
Go back to the PEI connector. Check continuity between the black w/yellow trace pin down to the little plug that connects to the spark coil. Once again, continuity from the pin to frame is bad.
Check the Black w/white pin on the PEI connector to frame. This one you want to find continuity. If not then the PEI isn't grounded properly.
The Service Manual has a section on the proper continuity checks and ohm readings for the PEI, spark coil, and magneto. This can help determine if those specific parts are bad.
Also here's a wiring diagram that is color-coded. It's a bit easier to follow than the black and white one. (edited)