Love the original pvc tank very inventive. Don't throw away your leaky tank, regular silicone from any hardware store will seal it up permanently. Had that problem on a motorcycle tank, the stuff is still holding since 1970. I still ride the bike to this day a 1969 Triumph.
I'd go the Ultra Grey, after a good cleaning with MEK or acetone. Is that a galvanized pipe cap for your gas cap?