I am surprised there is not already a thread on this, in fact I was kind of expecting a sticky! Esteban knows a lot about this. Having done x3 P engine rebuilds using P40 parts I am seeing what works and what doesn't.
What works:
head gasket - OEM still available from Nissan if you want to use a good one
oil pump - OEM still available from Nissan
fuel pump - OEM still available from Nissan
camshaft - OEM still available from Nissan
seals and gaskets - all work except for the timing case cover
oil cap and oil cap gasket - works just fine
distributor - all P40 dizzy's work
oil pan - works fine
pistons & rings - works fine
valves and springs - works fine
bottom end bearings - all work except for #1 which you need to use from the PPU engine. Esteban correct me if I am wrong here.
lifters & guides - work fine
What doesn't work:
anything time gear wise - P40 is a chain driven timing gear, case is completely different along with internal parts
carbs - completely different
What I am unsure on:
manifolds - no idea
head - as an assembly, I think it will work though
crank - no idea
flywheel - no idea
Please add to this what you know.
Cheers