[Nisgeis]

Well, hopefully the engineer would have done all the work whilst in the pen, where he could have the engine apart. There's little you can do whilst you need the engine to be available/running, other than inject more diesel. Diesels have a direct relation between the amount of diesel injected and the power produced, right up until the smoke goes completely black and starts ejecting unburnt fuel. That comes at a cost of fuel efficiency of course and if you run your engine at too high an RPM it can be damaged with joints parting / bearings overheating or melting etcetera.

In port, he could balance the crankshaft, polish the air intakes, check the injection timing, perhaps change the oil filter for a less restrictive one, perhaps if he wanted to change the compression ratio by capping the pistons. It depends what he had available to him. Things will come out of the factory to within a certain tolerance and can be improved.