I would use a 3/4” right angle. See if you can get it in Marelon material since you don’t want it to fail. If you can’t find that , just get it in nylon. I would not use PVC which is too brittle. When I replaced all my fresh water connections, I got them all from usplastic.com. I used FDA water safe nylon fittings. They are milky clear in color. Can’t remember the exact line at the moment but they have endless lines and hundreds of sizes and angles etc. Hope that helps. 

I did the same job in another model Catalina boat.   I used a Marelon 90 as linked below.  It worked and fit perfectly but the the 90 would continue to thread into the tank.  It didn’t have a stop to tighten against.  I used a locknut as shown in the second attachment.  It allowed me to point he 90 in the correct direction and fix in place.    I don’t know if this is the proper method but it did work fine without leaks or odors.  

Good luck

https://www.go2marine.com/Forespar-Marelonsup-Male-NPT-to-Hose-Barb-90-Elbows-HECM?quantity=1&diameter=1002

https://www.productsforautomation.com/sealcon-npt-locknut-nn21bk-p/nn-21-bk.htm

You might also consider replacing the existing vent with a mushroom out the side and increasing the size to 1". Less smell by increasing the o2 in the tank.