I too had a similar situation on our JH4 non turbo so I spoke to our diesel
repair shop who does work for our company. Their recommendation was if smoke is "mild" he'd recommend an additive....he suggested Stanadyne. He says the stuff cleans injectors and it has cost his company a lot of new injector business but he feels it's his duty to let customers know about the stuff. We now run it and I have seen improvement.
If you don't trust additives Id take them to any good diesel
shop and have them tested. Like one of the other writers suggested, boat injectors do not really work that hard and in most cases a simple cleaning
will fix whatever ales them. I would not buy new ones and be very concerned of any shop wanting to charge $100 to "repair"