Decided to install the oops fitting, rather now than later. It's not 100% clear how these oops fittings are to be installed on the old Onex tank, but I hope I got it right. According to revision B on ONX-G01 from 12/03/2015, is says:
Added Oops fittings for fuel ports. Use of Oops fittings requires moving fuel supply port to bottom of tank sump. Removed optional fuel sight gauge. Added warning to NOT slosh fuel tank.
So I made a big new hole in the tank, with a tool that looks brutal enough for a picture. In hindsight I could also have up-drilled the hole for the sight gauge to 1 inch, but there is no way to look inside the tank and see how things are before drilling a hole, so it's a catch 22. Can't see that the exact placement will harm, it's pretty sturdy in that little box at the bottom.
These oops fittings are not straight forward to install. They require considerable trimming on the inside, and it's almost hopeless to get there. David Amsler has made some super smooth tools for this job. The tools are sent around the community (very nice gesture), but me living in Norway, the shipping will cost a lot. He gave me some ideas though, and explained a lot of the job, regardless of actual tool used.
To fix the 1/8 inch NPT hole I updrilled to 10mm. This was my largest drill. I then reamed it up using adjustable reamers to the correct size, 9/16. But then comes the really hard part, that is to fix the inside so the oops fitting have a nice flange to seat the O-ring. That is where the tools from Amsler comes in, and makes this a piece of cake. I wondered how to do it without the tool, but then I found this adjustable wood drill. The edges of it is very sharp, also on the back side (the side towards the drill), and the diameter is about 13.5 mm, fitting nicely inside the 9/16 hole.
This worked just fine. The edges became a bit rough, but nothing that a small Dremel disk mounted through the hole couldn't handle. The result was a perfectly smooth flange on the inside.
The 1 inch hole works just fine with no trimming because there is no insert to drill out. Mounted everything, just to see how it fits, and it all looks just fine to me.
All in all, looks OK. I need to clean the tank, and then test for leakage, and hopefully it should be OK. I made this push-pull fuel valve knob with support. This will be redundant now. I need to make a turn handle instead, out of tube or something.