Sorry to see that. The hole might be too big. I have POR-15 in 2 of mine and they are still good after several years but the holes were smaller.

What a bummer 

I'd clean it up around & out from the area then solder it with good flux & a very hot big soldering iron.
[Then go looking for a good tank online].
Was a fairly regular problem at work with older bikes especially farm bikes & was off down to the radiator shop & he had it sussed each time.
It'll be bad all along the bottom so no winners really just "get you by for a while methods" 'i think'.
The good thing is "it won't be visible under there,

Ok, so now I'm a little nervous. Last time I rode it and noticed the tank leak,  it was running fine and then like a flip of a switch it started blubbering and running extremely rich. Smoking like crazy and wouldn't get past 4k on the tach to try and clear it out. So while I sort out my leaking tank, I will slip a misfit tank on it and try to figure out what happened. I'm hoping the fresh top end didn't catch a ring on the ports and break a ring. Maybe the nasty tank passed some debris, but it has a new petcock and a new inline filter, so not likely. I am running premix along with the autolube, but it ran perfect for about 20 minutes then instantly fell flat on its face sputtering and blubbering. 

Schu how rusty was the inside of your tank?

It didn't look much worse than others I've cleaned up when looking thru the fill hole. But it must've been bad on the right side bottom surface. I did one 24 hour stint of evaporust along with multiple shake downs with strands of various size chain lengths for abrasives. A lot of loose flakes came out. That next ride was when I saw gas dripping onto the exhaust. I think my son has one of those bore scopes on a flex stem. That way I can get a good look at what I'm up against..

If it’s clean and you can get to the hole from the inside use some JB Weld tank repair 

That's unusual--ran good then instantly went big smokey & blubbery. On the face of it it sounds like the right side crank seal did something--fell out--got damaged etc. That's unlikely.
So more likely you hit a bump & a lump of stray carbon got lodged in the spark arrestor in the tail muffler. Then you are using full throttle to get along at a low speed so more oil & fuel ==smoke & no power==constipation.

Or float bowl o-ring failed? 

Very fine rust “dust” and the filter won’t allow the desired amount of fuel thru?
Sneezles61