I always just pulled the boat up to the pump and stuck the filler nozzle right in the tank. Never took it out.

You could take the tank out and bring it to a welder, have them cut a hole in it and weld in a pipe coupler, then install a hose barb, a short hose, and one of these in the deck, or wherever.

PERKO Gas Fill & Tank Vent, Black | West Marine

You could leave the factory cap in place as a vent.