That's not so terrible.
Have access to a MIG welder, a 4-1/2 or 5" angle grinder with plenty of good (Norton or 3M) cutoff discs and flap wheels @ 36-80-12 grit, aviation snips and either a hand or pneumatic panel edge flanging/5mm plug weld hole punching tool, a few dollies and body hammers?
There's loads of info on the intrwbzz, and motorbooks international sells a shedload of books on the topic.
The key is to work deliberately and carefully. No short cuts. Cut it all out, make nice patch panels of 18 or 20 gauge mild steel-
(1018, 1020, something like that - McMaster-Carr if you can't get it locally. Sheet metal shops will often give away a pile of off cuts, old appliance doors work well too)
Flange the edge of the area the patch is being welded into so there's maybe 3/8" overlap. Fit the patch carefully, tack weld it in several places while pressing it home firmly. Add more tacks, tacks between the tacks, then connect the tack welds 'til it's welded in. Alternate where you're working to avoid inducing distortion due to concentrated heat.
Grind the welds w/ that 36 grit flap wheel. Again-mind the heat; a coarse flap wheel can burn 18 gauge steel blue pretty quickly. Get the high spots off, blend it fairly well, and get the first coat of filler on.
Shape the first coat or 2 of filler w/ 36 grit on a flat board sander. Next coat or 2 get 80 grit. If it's close switch to glazing putty which is creamier filler (and which hardens in like 3 minutes - mix only a little, coat quickly and wash the spreader off with lacquer thinner while it's hardening) which is sanded w/ 180 grit. That gets 3 or 4 wet coats of primer which gets blocked w/ 180, a final prime blocked w/ 400 wet, and you're ready for paint.
I'm sure this seems totally anal-retentive, but I've done a lot of this stuff on both cars and many endurance race motorcycle fuel tanks I've made, and in the long run it produces excellent results (which last) the first time - which is easiest in the long run.