You could pull the tank off, drain it, drill a hole at the bottom of the crack, and one on the top of it (to keep it from wanting to continue up) clean the hell out of it, get some fiberglass epoxy and some cloth mesh, and epoxy the tank. The epoxy wont be as flexible as the tank is originally though.
Another fix would be to use some yamabond and flexible plastic to seal the crack. Id clean up the tank, drill my upper and lower holes (you gotta keep the crack from wanting to continue on), and then seal the holes with yamabond (any automotive RTV would work for this, just yamabond is better), and seal the crack with yamabond, let it dry, run a thin strip of flexible plastic along the crack, and yamabond that to the existing stuff youve laid. That would seal.
Or you could do like I did for my girlfriends uncle, I just found a nice guy on here, sold me an oil tank for less than 10 bucks shipped, and just replaced it. You could spend more than 10 dollars in time and money in repairing yours.
Another option would be to use this opportunity to ditch oil injection and get a blockoff kit, ditch the tank totally, and just rejet and premix.