I replaced a fiberglass tank on my 73 ducati that had started to soften in spots. I cut out the soft spots, filled the old tank with urethane foam and then finished with fiberglass and epoxy resin to get my plug mold. I then made two female molds from the plug, a top and a bottom.
I then used a fiberglass veil for the surface next to the mold and then coarse yarn carbon fiber cloth with West Systems epoxy resin to make the upper and lower pieces of the tank. I used a vacuum pump and bag to create the consolidation pressure that I needed.
At this point I simply epoxied in the hardware necessary for the fuel cap, attach points, and fuel petcocks. I then glued the upper and lower pieces together with filled epoxy adhesive, trimmed, and then used POR15 tank sealing system. At that point, it was time to paint.
Turned out great. Lots of work and a certain amount of expense. However, I wasn't able to find a replacement tank; so, it became a DIY project.