While enroute from Darwin to Maningrida at 9000 feet and at 12nm NE of Nabarlek, the pilot, who was the sole occupant, noticed a drop in the starboard engine manifold pressure. He looked at the engine and saw smoke coming out of the right hand side inspection vent. The manifold pressure had fallen to 15in Hg and by this stage, oil was starting to leak from around the starboard engine cowl at a considerable rate. The pilot carried out the 'engine fire' procedure drill and secured the engine, advised ATC and diverted to Nabarlek for landing. The propellor continued to windmill with the pitch lever in the feathered position until the aircraft speed reduced to 90 knots IAS on final. The propellor appeared to be about 90 % feathered. The aircraft landed safely. Engine damage was caused by failure of the crankshaft counterweight flange allowing the counterweights to separate. The crankcase was damaged resulting in a severe oil leak onto the turbocharger causing the smoke seen by the pilot.