You will have to take the engine out of the frame first, then take off the rocker cover,
then undoe the tappet adjusters fully now you should be able to get one side rocker shafts and arms out for inspection ,
i can't remember if you can do both sides at once or if you have to do one side then put it back together to do the other side?,
you may have to rotate the camshaft to undoe the other side? hope this helps cheers Mick.