Twin or single? I thought 20.5 was biggest single but not sure.
Either rocker arm stud loose and/or bent push rod, on twin-push rods bent / jumped off rockers-possibly caused by slipped valve guide from over heating, or popped out intake valve seat.
I just had to pull one off mower and take crankcase apart to remove bent pushrod ingested inside after it slipped exhaust valve guide bent pushrod and jumped off rockers-possibly then found its way into block.
