Not many Bronco 2's had the roof rack option I don't think. I've never seen one in person, but I don't think they were much for carrying anything at all. I thought someone had mentioned somewhere like 150 pounds, then again its a short narrow wheelbase vehicle, so adding weight up top isn't a good idea.
Only thing I could think of would be to modify the current cross rails you have with some angle iron or square tubing maybe? Use the current mounting brackets and fabricate new cross rails. I can't think of any vehicles that use the same rack system either so trying to get some extra parts to mess with would be a pain too.
May have to completely re-invent the wheel with a new roof rail system and build your own kayak mounts yourself....just make sure you seal all the holes in the roof really well.