Hi, Chris -
I did some Internet research on the Cal 2-27 before responding to your post, since I've seen a couple but couldn't recall whether or not the halyards are internal or external. I also read the old Practical Sailor review of the boat. None of the references I found, though, said if the halyards were internal vs. external.
Since the mast is deck-stepped, here's one way to tell: if the halyards are internal, then there has to be some sort of exit for the lower end of the halyards somewhere on the mast. It would most likely be a slot or gate of some type. It could be a little ways up the mast (but certainly below the spreaders), if your halyards are rigged to be hoisted at the mast; or it's possible it could be down by the base of the mast, particularly if your boat is set up with the halyard running back to the cockpit. I had a Catalina 30 once that was set up with internal halyards that way - it was a custom mast set-up, not the original.
If you don't see an exit slot for the halyard, then the halyards would have to be external. The original spars for the Cal 2-27 were made by either SuperSpar or Kenyon, and the Kenyon masthead setup had 4 halyard sheaves (two foward side-by-side, two aft) to allow for two halyards, side-by-side. There was some good info and pictures on this at the rigrite.com website (standard disclaimer: no affiliation with rigrite).
If this is how your boat is set up then my guess would be that the halyard was pulled to the top of the mast, and the end then slipped off one of the sheaves and then back down the mast. If that's the case then you'll likely have to either drop the mast, or go to the masthead to run the halyard back through both sheaves. As long as you're doing that, if the masthead has a second set of sheaves, it might pay to go ahead and run either a second halyard, or a messenger line that you could use to run a second halyard if you had to.
BTW, in my research I found quite a bit about the Cal 2-27 on line, including some good rigging info, so I'd recommend you check some of this out if you haven't already done so. In particular I saw mention of wooden spreaders that may need replacement if rot is present, and how to paint/varnish them to prevent and monitor for rot; undersized standing rigging that you might want to upgrade; undersized or non-existent backing plates on deck fittings and hardware; and underpowered engines with some problems getting spare/replacement parts (if outfitted with the original engine). That's just a quick list. Overall the boat seemed to get pretty good ratings.
Hope this helps - it's an armchair answer based on my experience with internal and external halyards on other boats and my research. Hopefully some 2-27 owners will chime in with first hand info. Good luck!
Mike Turner
Lazyjack 32 schooner "Mary'Lis"
Mobile Bay, Alabama