They make a two-part putty epoxy stuff called, surprisingly, Gas Tank Leak Repair. You can get it at most cheapie auto parts stores.
Pinch off a bit of resin putty, pinch off an equal bit of hardener putty, knead them together into a kind of chewed-gum wad, and stick it right on the seam. No other preparation required, other than not really loose dirt or flakes, etc.
I've used it two or three times on external seam leaks. Couple old bike tanks, and an old car tank. The car tank was still damp with fuel at the time. All worked perfectly!
I originally picked up the stuff just as a lark one day. What a surprise to find it actually works!!
Good luck, and neat bike, by the way...
Kirk