The Calypso VI is a very basic piston design and the second stage parts are fairly generic. I can check for you this weekend though on the availability of first stage parts and to be sure about the availability of second stage parts.
I also vaugely remember a recall of the Calypso in the late 70's or early 80's, mostly because of the concern at the time that it could fail closed (a major rarity in reg failures) and partially because it confirmed the belief at the time that quality control at USD pretty much sucked - it was a bleak time in their history and they had a reputation for generally poor quality during that period.
I don't remember the specifics of the recall or the fix, just that it was a big deal at the time and warranted a lot of critical comments at the local dive shop (a Scubapro dealer).