It should have a silly sump guard as stock which is best junked .
Crash bars are standard- the silly flip up side stand fits to them. Most folks fit a aftermarket stand, anything is probably going to work better than the stock item.
Metz Enduro 3s work well with the bike and ccope well with deep sand and mud. Mitas E09s are tough off road tires if you can afford a set just for the two day trial.
My rims dont hold air and I use tubes but not hard to check if yours do - just give the bike a good thrash down a bumpy and twisty road and if they go flat you need tubes. Thy usually break away from the bead when they do, so they take a bit catching.
Nothing wrong with the forks except a little soft, 10 mm more preload on the stock springs and 15 wt in the compression side and 5 in the compression work OK for some. Moxima or Belray Synth suspension fluid works best, Dino fork oil went out with Dino engine oil.
HPN inserts work better, but they are NLA.
Front brake works fine with HH pads, either EBC or Brembo's own if you can find them. Lucas shoes are good in the rear
The stock Ohlins with a 450 lb spring works excellently , Wilbers are reputed to be OK and are a bit cheaper- Richie Moore sells them and will know what works with the GS.
Around 60 mm rider sag at the front works with the Ohlins at close to stock settings.
Never had a problem with diode boards in half a million ks on an Airhead, just dont jump start them and buy a quality AGM battery.
If you must change EME have a modern reg/rec complete with a harness for around US$100- ,and a few other goodies too.
Centre stand gaurds, Barkbusters, relocated oil coolers, bar risers, lowered pegs, etc, etc are fashion items and tou may find the bike will works fine without them
Even with the stock pegs the brake and gear pedals dont work well with my size 12 Gaerne off road boots, so you might have to to a bit shortening/ extending to make the work, and you will need majer worh with lowered pegs.
Stock ignition system works fine, Boyer Micropower system has a better digital curve but takes a little fiddling to fit into the hall effect bean can.
I found that higher top/ lower first gear conversion are unnoticeable on a properly tuned bike but they seem to be popular.
Never found the need for more watts than the stock alternator puts out, but if you plan on fitting a heated bidet on the rear carrier or whatever -----------!
But Ianb's advice is the best off all , before you do anything ride it as it is - they are perfectly acceptable bikes stock and after a few months you will know what, if anything, needs to be changed to suit you and the use you put the bike to.