Kohler XT650 help needed (Not starting)

[Nico F]

Hi. I have a Kohler XT650 lawn mower engine that wont start. When i first got it it wast turning over, but after putting in an air filter and a new spark plug, I can hear the low rumble of the engine trying to start. It was left outside over the winter with a low of about -10 centigrade, and has been sitting for a few months now. As I said, I replaced a spark plug that I tested to have a break in the line, and put in an air filter where there wasnt one. It had looked like the mower had been run without the filter because when I opened up the carburator I found tons of gunk in the way that I quickly removed. My final guess at the moment is something wrong with the compression chamber (leak, cracked piston head... idk) but I dont really want to open up that area of the engine. Are there any suggestions or thoughts on what could still be wrong with it? If not ill just try to sell it cause its a pretty nice mower. Thanks