I'm confused. To my way of thinking the hatch covers are nearly square in length and width dimensions, but it sounds like you are referring to the long boards running down the length of either side of the floor below the gunwales.
If it is the latter, I would look into azek or pvc trim boards if you can match the length and width.
I think this would be far less expensive than trying to find replacement boards for the boat.
For whar it's worth, I have used this material for boat projects like making bases to mount electronics for over 20 years with good results.