I have been following this for a bit. I have experience replacing the mast step on a smaller O'day 192 with Z-spars mast. It, too, had the foot casting with integrated sheaves for internal halyard routing. A replacement kit was sourced from Rig-Rite, and it did not have the original casting. As Jackdaw explains, it seems this integrated sheave casting wasn't very good. The replacement kit did include a tall plastic block to shim the height of the old casting. We also had to order stand-up blocks mounted with nutted studs that go into the deck plate, which sits on top of the spacer block. Side exits ports had to be cut into the mast to allow internal halyards to exit, and Rig-rite had your choice of plastic or alloy exit trim plates for these holes.
The original base was broken when my friend brain-farted and tried to step the mast without all shrouds connected, so it went right over sideways and broke the pivot pieces. Apparently when unstepping the mast this fall, he was unstable on the lowering, and broke the pivot pieces AGAIN. So, no matter what you do, be REALLY REALLY careful on hoists and drops with these cast step pieces. Given what I've heard of stepping masts on these bigger O'days, I don't think I'd attempt it without a well-designed set of baby stays and A-frame or gin pole.