Pull the plugs and squirt a penetrating oil in there- real well. Let it sit a day, then do it again. In a day or so, hand-turn the engines a coupe of revolutions. Then crank out as much oil as you can. Change the engine oil. Pull the pumps and look at the impellers. Check over all the fuel lines & electrics. Pump out the fuel from the tanks then fill them at least half-full- or run the engines from a remote fuel source. New plugs, filters, etc. Fire the engines & watch for obvious issues. Warm them up then run a compression test. That should do it.
I'd also take a good look at the outdrive boots.