David,

I can't comment on the valve service for the Yanmar, but I have pulled injectors on these engines, and that was a quick and easy job.  After 1000 hours they should be in excellent shape (unless you have had water in your fuel or other problem).  Testing them at a dedicated diesel shop should be pretty inexpensive, if they pass muster.

Bill Kinney
SM160,  Harmonie
Hollywood, FL, USA