engines are simple. They require metered fuel and wide open air for proper operation. If either of these factors are altered, the result is incomplete combustion which is black or white smoke. Check the air intake to ensure it is clear. If so, remove the injector nozzles and have them serviced. They can be rebuilt for a fraction of the cost of replacement.
There are other possible problems such as a failure of the injection pump
and/or improper timing but they are less likely.