The valve lash on your mower is via an eccentric bush within the rocker
The rocker bolt should have a square section on the shaft that engages with the bush.
This set up was invented around 80 years ago to reduce the weight of moving parts thus lessen vibrations & allow faster rocker movement.
If the bolt is in the bush correctly andthe rocker is not damaged you should be able to get around 1/4" of adjustment at the vlave stem.
If this is not enough then either the rocker is faulty or the valve stem has shifted or it has the wrong valve fitted.