AIUI, everything above the crankshaft should be interchangeable between JP2 to JP4 (was there a JP6?) models - i.e. rod, piston, rings, barrel, head, injectors, cap & nut.
Fuel pumps will be different, the crank obviously, possibly the flywheel is bigger on the JP3 vs. JP4 (I don't know). Oil pump may be the same, again, I don't know for sure.
Which bits are you looking to replace?