A quick test of the rest of the tank is patch the hole using chewing gum and duct tape, get some small stones (preferably sharp, locally sourced and organic) roughly quarter fill the tank with white vinegar.
Seal any other holes with duct tape before vigorously shaking for 5 mins in 1 angle. stop, get your breath back
rotate the tank and go again till you've given most of the tank surfaces a good kicking.
The vinegar helps remove corrosion but the stones will act like media blasting. I know vinegar won't remove the rust like stronger acid but if there is a risk of further holes you don't want to give yourself chemical burns.
reassess the tank and go from there.
I did this with my cortina tank (I didn't film the stones bit as I looked massively out of shape!) but the stones did create a few pin holes which meant I had some thin steel which would have been an issue further down the road.
Pinholes were repaired using plumbers solder as it doesn't heat the offending area as much as MIG welding and you can reduce chances of your tank exploding (not that fuel is an issue in your case)
As for the patch I'd assume 5mm or so periphery around the hole for bad steel, you could attempt a patch and tack it if you feel confident then drop a few more tacks on then solder the rest but use a little more heat application so you can get liquid solder into the gaps.
Solder can be filed simply but I didn't go wild with it as it'll be as bad as thin steel.
Sorry if it sounds like I'm telling you how to suck eggs but I've done this on 2 tanks before and it seems to have done the trick. The first tank failed because something went wrong with the POR15.
EDIT: where are you in the world?