Are you getting a good, strong spark when testing from the plug wire toward ground (bypassing sparkplug) ? ....The spark should be able to jump a 1/4" gap.
Measure the resistance of your sparkplug from the terminal to the center electrode. .....Are you using resistor-type plugs?
Yes, a new magneto could be defective, or weak. .....I don't remember if you replaced the armature, or just the coil. ....Is the coil loose on the armature? .....Does the coil have a good ground?