(x-posted to DiY)

I have an old motorcycle gas tank that's in need of serious repair. It's a saddle-type tank, where the frame top-tube divides the tank in half, with a separate fuel tap on each bottom. (Thankfully, the areas around the fuel taps are sound.) Most of the bottom of each half is rusted out, and there are small rust-holes going up one side about 1/3 of the way. I'll be painting the tank, once it's repaired.

I know -- the first response from people is "get a new one", but this is an extremely rare bike, and after 6 months of serious searching, I have not been able to locate another gas tank anywhere in the world. So replacement isn't an option. My budget is low, so having it professionally restored isn't an option either. That leaves repairing it myself. Most of the bike's restoration, I'll be tackling myself and, having restored (to at least a decent amateur standard) a dozen or so bikes over the years, I'm pretty confident about doing whatever is necessary. However, I've never done rust-patching where it would be visible AND where it would have to safely contain gasoline. That's why I'm asking here.

The options that I know of are forming steel to the right contour and wire-welding or brazing it in place, and then doing any necessary filling with lead, brass or 2-part filler. I don't have a wire-welding rig, so that's going to make it more complicated, although I'm considering buying a flux-core (gasless) welding rig. Or, I could buy a brazing torch rig and go that route. I've done acetylene welding in the past, but never wire welding or brazing, although I'd like to learn and think I could pick either one up pretty quickly.

Are there other options I don't know about? Is there any way to use say, fiberglass or aramid-reinforced resin that will be able to patch a bike's gas tank? Some new miracle material or process?

Thanks in advance for any advice.