I'm a novice at mower repair, so please go easy on me! My adventure started when I bumped the head cover and dented it. The engine popped because the input valve couldn't move. I banged out the dent and found that the tappet nut popped off. I replaced the nut and set the gaps according to the motor spec and put it all back together. It worked great for a few minutes when the engine seized. I had neglected to re-seal the cylinder head cover and apparently enough oil leaked out to cause it to seize. I got some liquid gasket to seal that, picked up some PB Blaster, removed the spark plug and soaked the cylinder for a couple hours. I still couldn't pull the starter so I removed the fan cover and tank and managed to spin the fan manually to un-seize the engine. I put it all back together, pulled the starter a couple times without the spark plug to eject the PB Blaster and replaced the spark plug. It started up again with a huge cloud of white smoke. After a few seconds the whole thing went bang and now it won't start again. The engine isn't seized, but I don't think I'm getting a spark. The starter feels a lot easier to pull than normal, so I'm really afraid I broke it beyond repair. Any ideas?