At my normal hourly rate it would have been cheaper to buy a new gas tank, considering all the research, driving around town and mail list correspondence. But "saving money was not the point -- conquering an obstacle in repairing my gas tank was. I hope to never have to do this again but I'm prepared if I do...
This is the crack before I did anything. Notice that the crack runs under the petcock O-ring and almost to one of the brass inserts.
First, I routed the crack wider with the smallest metal bit I had for my Dremel Mototool, then filled this now wider slot with Devcon Welder II epoxy. An hour after applying this epoxy, it seemed like a success, as it was sticking to the plastic. After letting it sit for 3 days, though, the thin stuff on the perimeter was peeling off. Put gas in the tank and it uniformly flowed out of the repair. This is the crack with Devcon Welder II epoxy just before putting gas in the tank.
Next, I bought a "hot air plastic welder" from Harbor Freight. Didn't have the appropriate welding rod for polyethylene, so I cut up a bucket that scoopable cat litter came in and practiced on that using slivers of the lid for "welding rod". Made a minor mess, as the material was thin, and the air from my compressor pushed the plastic away. Didn't want to make the same mess of my gas tank, so I cleaned out the old epoxy and routed a bit more with the Dremel to make sure I had clean plastic once again. Didn't take pictures of the tank as I had my hands full.
Finally, I ordered a metal tool from Urethane Supply designed to push the plastic around and heat at the same time, and that did the trick.
With the hot tool and already widened crack, I made it even bigger so as to melt all the way down to the inside. Kept the tool moving in a small circular pattern so as to mix the plastic. Fed the provided polyethylene welding rod material and procceded to fill the trench I had made and mix the plastics. Only a little of the welding rod material is visible from the outside as making the trench pushed Kawasaki green plastics up. I used the hot tool to plow that back into the repair.
After letting the repair cool to the touch, I used the hot tool to reshape the petcock recess. Wasn't all that hard, and it held gas! No cutting or sanding. I poked the reflowed plastic of the repair, and it had about the same waxy slick feel as the rest of the gas tank. The little black specs are burned plastic which was in contact with the hot tool a bit too long.