Provided this is repairable by welding, you will need to get this area very clean. Read some of these posts in past discussions and get a feel for the degree of cleanliness you will need to acheive. Being a diesel combustion chamber, my guess is that you will have your work cut out just preparing this area for welding. What welding process are you planning to use to make this repair?
Here is the link to the posts I was talking about...
http://aws.org/cgi-bin/mwf/topic_show.pl?id=2529#8291
You will have to scroll to the top of the page to get to the start of the discussion.
John Wright