I know they use the following products for motorcycle gas tanks: "Kreem", POR-15 and JB Weld epoxy. But I'm not sure about plastic/fiberglass vs steel tanks. I know some have used Kreem in Norton FG gas tanks but don't know how successful/long term the fix was.

I'm a little hesitant to give this advice but here goes.... Your tank is most likely Polyethylene, which is a thermoplastic that can be repaired by melting and fusing the tank at the seam. The easiest way to do this is with an electric soldering iorn (used to solder electical connections). Use a cheap iorn with a flat tip. REMOVE AND FLUSH THE TANK WITH WATER BEFORE REPAIRING. ALLOW TO DRY COMPLETELY. Once the iorn heats up, "drag" the tip over the seam, melting the plastic as you go. A couple of passes will result in the seam being fused together. Keep an eye on what you are doing so you don't melt completely through the tank.

That plastic tank is either polyethylene or polypropylene. Neither of those plastics can be repaired with adhesives, won't bond. Welding is done with a special hot air gun and filler rod, using technique similar to oxy-acetylene welding. Correct identification of the plastic is essential as is the use of the correct filler rod. Check with the welding instructor at your community college.

If you don't add a filler, you don't need to identify the plastic. If the leak is at a seam, there is most likely plenty of plastic to melt and fuse together. I have done this many times on chemical containers with a solder iorn. Like the previous posters pointed out, it is essentially a welding operation and both sides of the seams have to melt. A butt weld is out of the question without filler, but a seam, or flange from a seam are pretty easy to mend.

I would never repair a fuel tank unless a replacement was no longer available. I just replaced the fuel tank in my JD1070 and it cost a whopping $430. (ie one pinhole leak)

I keep my tractor stored inside the barn and I just wasn't going to chance a leak even though I knew that my repair would be 99% as good as new. It was that 1% that I was worried about.

Weld (fuse) it with a cheap soldering iron - no problem. If you screw it up buy a new tank. If it works you have learned something. I would put money on that the weld works and will last a very long time. If have welding experience you can also use a small propane torch to weld (fuse) the seam.

Here's a follow-up to the topic.

The mower is at my cabin and no electric available for the soldering iron. Bringing it home was not an option and a new tank was not cost effective.

JB weld did the trick! A trial spot showed that it did not provide a chemical bond, but it did take a hammer and chisle to pry it off. It filled the seam and over both edges completely, so that should add some mechanical bonding as well. No more leak! It's gasoline resistant so it looks like it should hold for the life of the mower.

Glad you fixed it. Just keep an eye on it.