Hello I'm new to the site and to mowers. I picked up a John Deere JS36 mower off craigslist and it's been running great for about a year until yesterday. I took the front wheel off for a minor repair, then realized the air filter needed cleaning so went to the store and bout a tune up kit with filter, oil, and spark plug. I had recently changed the oil so I was holding off on that but swapped the filter and the plug and started it up. A little black smoke came out, but stopped after a few seconds. In cut a few rows of grass but it didn't seem like it was running right. I investigated the filter and found it wet with fuel? Not sure why/how this happened, but cleaned that area and proceeded to cut two more rows, but still seemed like it was running rough so I shut it down. I opened the dip stick and it was smoking a little in there (which may be normal since it was hot), but I then decided to let it cool and change the oil since it did seem low now at this time. I then proceeded to crank the motor and nothing. I can't even pull it one inch. I was afraid I seized up the motor so basically took it apart with hand tools, except for the flywheel since I don't have a puller (yet). I did, however still see a flywheel key after i took off the 15/16 inch flywheel bolt, but not sure about the bottom half (i did not sheer anything). But got the valve cover off and see the piston and valves and i can turn them by turning the flywheel so my questions are now what? The piston and valves look ok from the outside except one minor nick in the piston. I'll post pics later, but any ideas of what is going on? I'm lost but would rather learn how to rebuild this motor if I have to rather than bite the bullet and buy a new $400 mower. Please help and thank you.