Best check your information The HH80, 100, 120 were cast iron. The HM 100 was aluminum and used a cast sleeve as was the H70 which originally had an aluminum bore. All these were used on MF lawn and garden tractors.
Sound to me as if one of the safety switches is not operating properly whether the switch is bad, out of adjustment or a broken wire. Use the trouble light or multimeter to trace the bad switch or wire
Here is the ariens site for the manuals for your machine. I tried to attach the parts book...