Cam chain, tappets or maybe something else...i.e. cam shaft bearing wear Tappets are easily adjusted. Cam chain can be replaced. Cam shaft / head wear is not good (maybe results in new head). Are you sure it's not the con rod or pin?
Make sure your getting oil in the head - there is small bolt on the right hand side you can slight unscrew while the engine is running - oil should come out of it. See owners manual or tech manual on this forum for info.