The way to do this with an alloy tankwithout making internal formers is as follows
1) make up a packing piece that is a tight fit between the two sides then strap the tank to it
2) block off the fuel outlet with a plug
3) fill the tank with water & block off the filler cap with cling wrap ( pallet wrap is better ) then close it
4) Hold th tank so the dent is upper most
5) Heat the edges of the dent going round & round, this will take a long time, probably an hour or more
As the water gets hot it will expand and because the dent is hot and a little softer than the rest of the tank, it will roll the dent up.
As this happens move the torch to just where the dent curls in.
When finished set it aside till it is stone cold before opening the cap.
Once the dent is out , it will have stretched the alloy a bit which can be heat shrunk, but you need to be really careful that you don't burn a hole in the tank.
Any one who makes aluminium car & bike parts can do that bit and you are probably better off sending that bit out.
Check out Kent White's site TM Tech
but be careful.
Sheet metal forming can be very addictive
If you have the time, take the tank to one of his courses so you can pick up a new skill & get the tank repaired at the same time