Could have been a piece of cloth someone had stuck in there to keep the rain out, who knows? I suggest you go through a rudimentary "safe to run" check on the engine. Disconnect spark plug and crank it slowly, to see if there is any resistance at some point in the rotation, or any odd noises. Turn it backwards as well as forwards. If all seems well, crank it faster. If it still seems OK, feel the compression specifically - most valve gear problems will cause an obnormality there. Look for oil leaks and check oil level. Check that it has fuel and the fuel is not going anywhere it shouldn't - not leaking or flooding. Then, reconnect the spark plug, start it, and run it at idle for a while. If it sounds good and runs properly, increase the speed a bit, with no load. Don't get carried away with the speed - I'm not sure it has a governor. If it is still running smoothly with no smoke or odd noises, try applying some load (i.e. mow a bit).
