Well that is no crack, that is from dragging the kayak.
I am sure the guys at OEX could fix it for you no problem, at least then you know it is done right.
I am sure they also have some plastic from cutting hatches that will work, maybe they can mail it too you.
You can also call OK and get some either from hatch cutouts or they also have welding rods.
http://oceankayak.stores.yahoo.net/weldrod.html
Here are some instructions on how to do it.
http://www.oceankayak.com/product_support/repair.html