I believe the AMF is from the mid/late 50s. AMF switched to a plastic steering wheel on their later tractors. You might find some Murray catalog pages and ads on ebay to narrow down the time period that model was produced. A similar Murray is called the "2 Ton" tractor where this one says "Super Diesel" and has the same rear wheels and flat grill with simulated head lights. You're right, the Murray came with a simulated motor facade and rubber spark plugs/wires that went to a simulated distributor. I have a 1958 Murray with sound device. It simply consists of a metal strip that rubs against either the sprocket teeth or the chain. A previous owner fiddled with the metal strip on mine so that it always "runs" quiet. I remember one of the pedal car parts sites used to have repro spark plugs...can't recall the site off hand. The motor facade will be a bit harder to locate.
Dave