I’m presuming it’s a GRP boat…
If the crack is just that – a crack – ie the thing hasn’t distorted at all, then I’d just go along with a small grinder (eg Dremel) and enlarge the crack into a V shaped groove.
Then get some SP epoxy resin and mix it with 50/50 micro-fibres and micro-balloons to make a strong filler which you can use to fillet it back up flush. Finally, working inside the tank, use neat resin to stick one or more strips of glass fibre matting along the repair.
I think you can buy a general purpose repair kit from SP that has all of the above stuff in, for about ?15-?20.
If the tank has distorted downwards, then I’d cut a wooden prop to go inside, just too large to reach the bottom of the tank then you can wedge it in place to push the top back into shape. Then repair as above.
In general terms I guess the corner of the hatch opening is an inherent weak spot, but nevertheless I suspect it has probably had an impact there at some point in the past that has weakened it.
As regards the integrity of the buoyancy tank, a repair as above should be plenty strong enough – the usual weak point is the hatch cover or their seals.
As an aside, I’m just in the process of adding a load of removable buoyancy to the tanks of mine. I’ve noticed that our local “Morrisons” supermarket is doing an offer of 2 x 5 litre containers of mineral water for just over ?1, so I intend to get a trolley full of those next week to use (less the water) as cheap “cubitainers”. I can’t think of anywhere else where you can get 5 litres of buoyancy for less than a quid!