my miss fire continued

After more investigation I have noticed a ticking at the rear of the engine at the top. I now suspect I may have a problem with one of the lifters.

If this is the case how much of a pain in the rear is it to do?

If I do it I would like to clean up some of the wiring and pipes at same time, is there anything about best way of doing this on the site?

Thanks in advance for any info