Gas tank repair epoxy

try some Seal All its supposed to be gas and oil resistant,im goning to try some on an old atc tank and see how it works.

IF the plastic is not brittle from age/gas fumes, then the best is to have someone plastic-weld it. Or, you can buy the gun from Harbor Freight pretty darn cheaply too.
This failing, I have used original JB Weld, but my hole was such that I was able to build up the epoxy over & around it...mine was not cracked. Apparently your tank/crack is flexing, so you may wish to search for something more "modern" by now on the market, b/c once epoxy is on there, you will never get it all off.

There are some kits used to colf repair / bond automotive panels that might be what you are looking for. 3M makes a product called Automic previously known as Duramix. There are other manufacturers with similar products as well. They are two part and typically require a mix gun, those kits will run about $35-$50.

If you go the hot weld route realize that it's more than just melting the plastic together! Eastwood Co. has one called Fiber Weld or something that is a pretty universal fillter / bonding rod. Otherwise there are specific rods based on the type of plastic you will be working with. Also realize you need to have a clue how to do this as melting the plastic to the point where the oils release will actually make the plastic NOT bond.

Since the plastic has been contaminated with fuel it may not work at all so be prepared to go buy a replacement tank from IMS or Clarke or another source.