gas filler neck replacement

Just use the new cap itself and leave the old part that it screws into connected to the boat. If its the same threads/size. I'd take the old hose to the parts house or local boat repair marina and size it up that way.   I bet they would have had the missing cap you needed without buying the whole deal.
Theres always a marina or boat repair shop close by that stocks the parts your looking for without online shopping. So make a few phone calls and support your local business owner.   Most of the time its cheaper than online and you don't have to wait on it.   Keep your local dude in business, you'll need him in a pinch someday.   Like my local guy will come in handy saturday morning cause I know he stocks about 3 different sizes of impellars. Cause the one I bought while I was in another city was to big, and I need the boat this weekend.   My wifes a hair dresser and all the girls that work there are supermodels and having a party at the lake this weekend and want me to pull them skiing. That boat will be running if I have to paddle it,,,,,