Iused some clear epoxy once on my KDX tank. It held long enough to get a new tank ordered. I don't think I would have trusted it as a permanent fix, however. Somehow 2.2 gallons of highly flamible material in my crotch is not that much fun.
My tank was cracked near the top cap (an area which experiences little flex/stress) when I got it used. Once I finally found the crack (I was sure it was the cap leaking, NOT) I patched it with JB-Weld and it has held great for over a year.
A new tank would be best, but if that’s not an option, try the JB-WELD. I drained the tank. Washed it out with Dawn dish detergent. Rinsed, Rinsed, Rinsed,… Let it dry. Washed with alcohol. Then JB-weld. Worked for me.
I have known of people who cleaned as above, then melt the tank back together (via heating a mettle rod, and using that to melt the tank). I am amazed that this hasn’t leaked yet.
I have also heard of (but never seen) “plastic welding”. Maybe someone else could shed some light on that.