Terry, go to your favorite auto parts store and buy the gasket material and adhesive that they sell for the fuel you're storing. Usually, the gasket material is available in several thicknesses and in large sheets; buy the thickest (to account for imperfections in the faying surfaces).
Try not to buy something which will rot (like the cork material unless they can promise that the resins that bind the cork granules will prevent rot spreading) or dissolve in or wick in water (some gasketing is basically cardboard).
You can double or triple gasket if you need more compliance. Cut the butt joints at an angle and fit them carefully, using the adhesive. I use the adhesive on only one of the surfaces so that I can imagine getting it apart again, leaving the gasket intact. Your tank lid should be well painted and the paint well cured.
You did not ask about the water tank lid gasket. Originally, mine was rubber. I reassembled the lid with ordinary house-type sticky-back foam tape (for thickness) and sealed the inside, waterside edge with 5200. The idea is that I could get a knife through the 1/8" or so gap of pretty soft stuff.
D