Just went thru this on my zero turn.. Seems a muffler leak caused the heads to heat up, and a valve stuck just enough to bend a pushrod.. Pop off your valve covers and check the adjustment...Also look for gasket leaks while you are checking things out..
My engine was toast as it needed two cylinder heads and piston/rings so I bought one online for $800 with a muffler, and am back to mowing. Equipatron has decent prices and OK service.