My last boat's Perkins 6.354MGTs and 4.107 powered genset hadn't been used in "a while" before I bought it. The PO reported 5 years. His neighbors said 10+ years.
The fluids were checked, the engine starters were bumped a few times, then primed, then started. Then cooling loop and turbos replaced. Then fluids changed. Then a great time was had by all (Well, the genset tool some repair work to the controller and massaging elsewhere).
Boat ran, and runs for the new owner, like a champ. Both mainstream had white smoke cold at the slip typical of the MGTs, one engine had a hint of grey at high rpms, but both were hitting spec for power and ran smooth.
The genset suffered a surprise failure of the raw water pump shaft, but other than that and some wresting with small nuisance leaks at the valve cover gasket on one, they all ran like Champs. Turn key and go.