Cracked as in won't be salvaged and the vent is available, or cracked as in the whole cap is for sale?I have a cracked roof with top vent if you are looking for that type.
My cap was good and rust free. Then I trailered it without the front cap being bolted down to the windshield frame. It blew up and cracked the fiberglass top about 2 feet from the front edge. The rest is good and the cracked could be repaired. I found a perfect one in Arizona and purchased it instead of repairing it.