Chris....I like the idea of yours fabricating an aluminum fuel tank to mount exactly the same way and similar dimensions as your original tank. I cannot see any downside except for cost. Aluminum welding should seal all metal seams quite well. It's certainly not going to be prone to stress cracking. And will look really sharp.
Alternatively, You could also consider rescuing the original tank, by simply applying a several layers of fiberglass with extra resin around the exterior, to seal all leaks and add strength to the current tank. Paint it red. Its certainly a lot lower cost than fabrication.