bandit 1200 forks are 43mm, but i think they are a bit shorter than yours, you can make them longer though, i did a thread on here somewhere, i'll see if i can find it !!!!!
you should be able to get a Bandit 600 wheel in your swinger (4.5 inch) with a milled down sprocket carrier and a slight offset front sprocket.
you might need the B6 calliper and hanger as well if its under slung or use a GS 500 calliper if its on top.
Ive used thin Viton rubber on some of my taps and blanking plates with no problem, but as said some thick gasket paper and maybe a thin smear of grey 3bond should work.
its not an exact science but it could be running a little lean on that cylinder, one of mine on my zed is the same but it runs sweet. maybe tweak the mixture a wee bit !!!
if you are doing it for the look then its not worth it for the small gain in power, good quality filters are expensive (don't buy cheap EMGO ones) a dynojet kit isn't cheap either and that doesn't guarantee it'll run right !!!! may need a dyno run as well at more expense. plus if you ride in the rain the filters can get wet.
if you mean the GS850L custom (chopper style) with the small square'ish tail light that sits on the chrome mudguard then no, the G is wider and sits in the tail unit.