I have repaired plastic gas tanks with a patch material from the auto parts store. It is an epoxy based patch with a piece of fiberglass cloth for the repair. Main thing to do is rough the area up with 60 grit sand paper so the patch will "bite" to something and hold. I would also do it to both sides of the crack.

The only problem I see is that the patch kit at the auto parts store is small so it may take a few of them to do the whole job.
Another option is to talk to a boat repair shop I am sure they have a product they use.

Good luck
Ron