Started the mower STC48V 24BS that has a 24 hp Briggs twin cylinder engine. It lacked power and, as I looked into it, one intake valve was not opening. The push rod was free and the adjuster and lock nut were lying loose having come off the rocker. I put everything back together but the intake push rod does not move it appears that the lifter is not moving. Any thoughts on what I should do next?