Rotax 503 Fuel Tank Fitting

Hello guys. I´m new to the forum and first owner of a trike with a Rotax 503. My engine is running very rough at idle and when I give it gas it will bog around 4000 RPMs. I noticed a lot of bubbles in the fuel line between the gas tank and fuel pump. Sometimes they will dissipate and re-appear and sometimes a big bubble will form at the highest point of the fuel line.

I´ve read many posts about air being sucked into the line. I pressure checked the tank and found an air leak at the tank fitting which I fixed. I´ve read about moving the fuel filter between the pump and carbs to eliminate possible air leaks. The tank is fitted with an old hose which I want to change to a copper tube since I´ve read is the recommended way.

Since I have never done that kind of job and the hole for the fuel cap is small and can´t fit my hand to screw a new fitting. My question is, which kind of fitting should I use? Or how do I go about screwing the fitting?

I have attached some pictures of the fuel line and current installed fitting. For the new fitting I have found in Aircraft Spruce a tank fitting with 1/4 barb end and 1/4 NPT, a pipe thread reducer and straight tube to male NPT adapter. Does this configuration works and how do I go about screwing the adapters inside the tank? I have also attached pictures for reference.

Thank you guys.