I and others before and after me have been able to nip the bastard up by either lifting bike or laying next to it and cutting and scratching knuckes and straining neck, got a open spanner between fender and swing arm to turn one flat at a time. Tire and fender off help some but not so much I bothered. I think sometimes the stud just spins in the horn, so must remove to epoxy or replace horn and cuss and strain it back on again. It might depend on how oil hose routed too. Oh yeah if you nip up too tight guess what happens, if ya don't nip up tighter than comfy guess what happens, ugh. I'd sure try to dabble mild locktite on it but pensive the tin metal will just beat some slack in again and only thing locktite does is make the job tougher yet.
If horn goes too whimpy the tuning nut can make in louder but must have horn firmly held down, like in leather jaw vise on stud/nut or it sounds so sick and lame its one of the more hilarious Norton experiences.