RE: that two-part "plastic putty" epoxy stuff:
^ I sold that same stuff made by Duro/Woodhill Chemical .
I'd go with the Seal-all.
Really sorry about stuffing my foot in my mouth there on the tanks. I had boxes full of parts and got rid of all of them a couple summers back and only kept a few.
I've repaired plastic lawnmower parts (and other stuff) with "Seal-All" so you shouldn't have an issue. I've had good results with it bonding to most plastics.
We have a mower grave yard two towns away. The entire front yard of an old house is covered with mowers.
I don't doubt it makes the town officials scowl because it's right in the center of town.
I could always take a ride there for a tank but, like I noted, I am just waiting for the right excuse to dump it, perhaps take it for a ride to that place.
I surely regret selling both my 2 stroke Lawnboy and the following Kawasaki powered John Deere walk behinds but I didn't imagine that future offerings would not be better. The John Deere had a pumped oil system and filter, so to think I gave that up for Toro. Big mistake.
Maybe stuffing peanuts along the tank edges could lure the rascal back to finish the job.