I also have a 3GM30
My suggestion is to find a good mechanic in your area. I noticed you are in Va. I am on the Chersapeake near Baltimore and know of some good people in Annapolis. Is that to far for you. I may be able to find some as far south as Solomons Island.
Most work on the engines such as rebulid of injectors etc are sent to Mac Boring ihorth Jersey. They may have a lead on someone in your area. The Yanmars are workhorses and will last forever when cared for and refurbished also. Do not give up on it unless necessary.