Peggie, the factory tank vent is about 1/2" ID max. and mounted on the transom (mine looks just like the fuel vent!). You could remove it, enlarge the hole, and put a open bulkhead thru hull, but it would face up, and take on water in the rain. I just removed mine, patched the hole, and put a new fitting just above the bilge pump thru hull on the side. Easy peasy. Someone here has some good pictures of the process-I keep forgetting to take them while I'm doing the work!Use a screwdriver blade, pliars or whatever works to knock the screen loose.
It's not hard to replace the vent thru-hull with an open bulkhead thru-hull. You need to remove a thru-hull from the INSIDE of the hull. There's a nut on the threaded barb..remove the vent line from the the thru-hull, then back that nut off the barb...you may have to scrape some bedding compound out from under it. Push the vent thru-hull out (there may be bedding compound under it on the outside of the hull too)...stick the new one in...apply new bedding compound on both the inside and the outside...tighten the nut, put the hose back on the new barb...you're done.
--Peggie