I'm no expert, but it would seem to me that the engine would have more value if you took care of things before a serious problem occurs, since it may be able to be refurbished/rebuilt and resold. I don't know if you wait until it dies if it can be saved in that manner without a lot more expense.
It doesn't cost anything to talk to an engine refurbisher in your area. See what he tells you about the value of choices. In fact, talk to a few. Just remember that a consensus doesn't make them right...
