Parts Interchange....
I *think* the CLUTCH from the Z32 (non turbo) is the same as the RB20. Other things commonly mentioned are spark plugs and oil filters.
I found a source in Austalia that stocks water pumps, timing belts, etc. Can not located the bookmark at the moment. I was reading the Skyline forum (Aus) and found a reference to them as a source of parts.
From what I have found to date THERE ARE NO SPARE RB PARTS stocked in the USA!

Water pump, timing belt, etc all are sourced from Japan or Australia.
ALTERNATOR.... need to check, but should be a common "two wire" L & S (lamp & sense) in addition to the "+" and "-" connection. As such any good rebuilder should be able to rebuild the alternator. Ditto for the starter. (typically only thing wrong with the alternator is front bearing - common item - and thre brushes. Starter -- solenoid contacts & brushes).
Tom :usa: