-- Last Updated: Mar-13-13 11:34 AM EST --
They may have some other types now, for all I know, but you say the rack is old so I'm betting you have their standard round bar. Others here will know more than I, but the Yakima accessories I've seen have all been interchangeable, fitting their round cross bars.
I sometimes mount Thule J-hooks (I have no idea what they are actually called) on plain round steel pipe. Thule bars are rectangular, but the J-hook clamps "designed to fit" those rectangular bars actually have half-round clamping faces, with a radius that's greater than that of any round bar you are likely to find. Still, they clamp to a round bar having roughly a one-inch diameter perfectly well. They can and will rotate (as the clamps slip) on a round bar, but that's fine because that means they can turn to a position in which they fit more tightly against the hull at a particular location (this is easy to understand when you see it, but not worth the long-winded explanation that might be needed to make it clear here). Once two J-hooks rotate to their best-fit position and are strapped to the hull, they can no longer rotate on the bar at all (the boat, J-hooks and cross bars, and the roof between the bars all become part of "one three-dimensional structure") and everything is solid.