Sorry for the delay I’ve just seen the post.
This happened to me on a Supa 5 where the best engagement I could get was about 10 to 15 degrees without a gasket, somehow it worked like this and stayed on whilst I was fettling it. The thread at the front of the port was chewed up but it looked OK at the back. The exhaust nut was in a bad way as well.
I removed an exhaust nut from another Supa 5 and this was undoubtedly better but not good enough, maybe half to three quarters of a turn thread engagement with a gasket.
Looking at the exhaust nut it was obvious that it wasn’t threaded to the end so I basically ground it back on the bench grinder until there was a good full thread at the end. I also bevelled the inside to match the exhaust pipe. I must have ground away about 6mm/quarter of an inch.
This now screws on about 3-4 full turns with a gasket and is rock solid.
Try it, you’ve got nothing to loose as you need to be fitting a new exhaust nut and gasket if you get the thread reclaimed.