I don't think sealer of any sort is going to work on it's own and I'd be worried if you remove the fitting with a holesaw that there isnt a huge flat area to put a new flanged fitting onto, it's not clear the scale here, but it looks like there's not much clearance between the nut and that shoulder in the tank.
if the existing fitting is flanged inside then could someone in the engineering capital of the country make up a threaded insert and a flanged nut, like you'd get on a tank fitting, but threaded to match your fitting. you could then put a fuel safe washer and mastic underneath the flanged nut and some sort of adapter to thread onto the shortened hose.
like a tank fitting, but without the flange.
View attachment 84469
Make sense?