Husqvarna YTH 2448 Riding Mower Won't Start?

It has a fully charged battery. The engine will not turn over and acts like it is locked up. The engine will turn a small amount, but not a complete rotation. Once in a while I can turn the key on and off in such a way that the engine "bounces" and starts, but it has quit doing that now. The only way I can start it now is to remove the left plug and let the motor spin a few rotations (spins freely with one plug removed) replace the plug and it starts right up and seems to run fine.

After I have mowed for an hour or so it seems to lose power, but still mows. If I drop RPM to an idle it pops a bit and if I pull the left plug wire there is no effect. (Plug not firing at idle). If I throttle up the cylinder it fires OK under load. Last year I had the dreaded bent pushrod thing and replaced both heads. I am wondering if one of the heads I bought on (eBay) was bad.