Under the engine covers two new Mercury Diesel engines were fitted with new counter rotating bravo 3 stern drives. This upped the vessels power from 440hp to 640hp. All the mechanical and electrical systems were replaced. The result was a boat better than new for less than the price of a new boat.
The other end of the scale is a simple outboard swap. The engine can be unbolted and the new engine installed. Often some ancillary items need to be replaced, or the owner feels like updating the rest of the vessel to match the shiny new engine.