You "might" be able to seal the crack.
RTV and JB weld WILL NOT work, don't bother trying.
Look for a product called Polysulfide. Can often be found at boating supply stores, or online.
Is is like RTV, but it is impervious to gasoline and other fuels... it will
seal an active fuel leak.
We use it daily where I work to seal fuel tanks, among other things. If you slap on a bunch of it and completely cover the crack, it won't leak (and will remain flexible)
This is worth a try... otherwise, new tank...