It's toast...
You MIGHT be able to heat weld the crack in the fitting, but the same stresses that opened the crack on the top of the tank will pull any weld open again.
Your best source for a replacement that WILL go through the companionway hatch is Ronco Plastics
Ronco Plastics Marine Catalog They make TOP quality thick walled (at least 50% thicker than most OEM and off-the-shelf tanks) rotomolded water and waste tanks for a very reasonable price (that's even more reasonable if you order through the online store at this site) and have more than 400 shapes and sizes, over 100 of which are non-rectangular. And they install fittings in the sizes and locations specified by the customer when they make the tank.
A tank that has enough capacity may require finding a new location and/or some re-thinking about installing it...I'll be glad to help you do that...just send me an email.