Stripped rocker arm on a TD

[Mark Sherman <MARKSHERMAN@worldnet.att.net>]

Problem: 
TD runs poorly.  Found no compression in cylinder #3.  Looked further and 
found adjusting screw loose on valve#6--the thread on the rocker arm 
seems to have disappeared.  
QuestionS:
1) What caused this? 
2) How difficult is it to replace the rocker arm?
3) Does this indicate more serious problems below?

Mark Sherman
'53 TD (supercharged)