The reason for the plug is it is very difficult to drill a curved hole. Any material that is gasoline and ehtanol resistant will do to replace the plug. Even a piece of aluminum with an o-ring or two around it will do the trick as long as it doesn't fall out.
Chemical resistance (4 is good, 1 is bad, 0 is unknown)
(Gasoline)ethanol
Aflas (3)0
Buna-N (Nitrile) (4)4
Butyl (1)4
Chemraz (4)4
Epichlorohydrin (4)4
Ethylene-Propylene (1)4
Fluorocarbon (4)4
Fluorosilicone (4)4
Hypalon (1)4
Kalrez (4)4
Natural Rubber (1)4
Neoprene (1)4
Nitrile, Hydrogenated (0)4
Polyacrylate (1)1
Polysulfide (4)4
Polyurethane, Cast (4)0
Polyurethane, Millable (0)1
Silicone (1)3
Styrene Butadiene (1)4
Teflon, Virgin (4)4
Vamac (1)0
Clearly, neoprene is a bad choice for sealing gasoline. Buna-N (Nitrile) is what I use for fuel system o-rings. I get them from Wesley Seals & Supply at 236 W Bedford Euless Rd Ste 4, Hurst, TX 76053, 817-284-3283.