Montgomery Ward Riding Mower Won't Start?

I am helping someone with their riding mower, it is a 80s model Montgomery Ward. I just installed a new ignition switch since the other was broken. It does not turn over or make any sound when I turn the key. I bypassed the ignition and started it from the solenoid with a screw driver and it started up fine, but it runs like crud and barely stays on. It has good power and battery and the terminals are clean and new. What could the problem be? spark plug? starter?