Welcome to the forum.
First, I checked Sony's recalls and your set does not appear to be one of the many RP sets for which they have extended the warranty and are doing free Optical Block replacements.
The info you're looking for is sometimes contained in the set's Service Manual. Try an internet search for something like: "Sony KF42WE610 Service Manual" and you will find internet vendors selling manuals. It would be nice though if you could make sure light engine replacement instructions were included in the manual before purchase. Perhaps someone else here can tell you for sure or has better info.
Replacement light engines are also available on line at better prices. Again, perhaps someone more knowledgeable here can tell you who is reliable and is selling quality parts.
More thoughts: brand new 42" lcd and plasma flat panel HDTV's have dropped dramatically in price and are headed down even lower as we approach the holidays. The technology contained in these new sets is, in my opinion, significantly better than the now-defunct Sony RP tech.
If it were me, I would not invest several hundred $$$ in repairing an "older" RP set like yours. By the way, repeat light engine failures are far from uncommon after replacement.
Yes, I know it's difficult to think about replacing an expensive TV that is only 5 or so years old. Unfortunately, that is reality in today's HDTV world. High quality flat panel sets made by first tier manufacturers
should do better going forward. Time will tell.
Good luck.
