If you can weld, or get it welded afterwards, cut the bottom out an inch or so in from the seams with an angle grinder fitted with a super-thin cutting disc, and tap the dent out from the inside with a soft hammer and a padded dolly, then weld the bottom back in again.
It's not a real hard job, and if you're careful, most of the original paint can be saved. (obviously not the paint around the crease) You may need to use a tank sealant afterwards though, just to make sure there are no pinholes around the welds.
I've had a couple of tanks that were repaired in this fashion back in the day when new tanks were "unobtainium" due to cost, and I repaired my GS1000S tank like this. (a PO had smashed the top of the tank in with his helmet when he was having an argument with his wife, apparently..........) The good thing is, it can't really get any worse..........