I Tenerised my girlfriends 1991 XT600E and she road it from UK to NZ overland. Not sure that all the mod's we did were essential, it depends if you are going to leave the tarmac to explore jeep tracks etc.
I would recommend fitting the larger Acerbis petrol tank, if only to avoid the constant worry of looking for fuel, and I would fit in-line fuel filters in the petrol lines.
I also fitted the Acerbis sump guard (engine bash plate) and engine crash bars to protect the sides of the engine in the event of a fall.
More Acerbis plastic was added in the form of hand guards that bolt into the ends of the handlebars, to protest the levers in the event of a fall, though having the lever mounts only just tight enough so that they can swivel if you drop the bike often saves the levers too. Heated grips were fitted and very appreciated at higher altitudes in Pakistan, India and Nepal, though again not essential.
I also got both wheels rebuilt with thicker spokes but reused the original rims.
Loud air horns are highly recommended in Asia, I made up a bracket no mount the compressor on the left hand side head steady, but wonder if this deprived the engine of cooling air.
I had Dave Lambeth fit his 5th gear up-grade kit which is expensive as the engine has to come completely apart, and I think he fitted an extra plane clutch plate to beef up the clutch. I also fitted an oil cooler via an oil thermostat (which I bought from Wunderlich, but they now only deal with BMW’s).
I fitted a K & N air filter rather than carry a dozen spare paper filters, but it was a pain to clean it. It you can find anyone who makes them I would get an oiled foam type, well I would buy 2 , the second easily carried with your spares and when you need to change the dirty one you can use the spare and clean the other at your leisure, not be stuck while you wait for it to dry.
I fitted Hepco and Becket pannier rails and used Darr Aluminium panniers, both performed ok. I had some supporting tubes made that ran between the foot-pegs up to the back of the rear sub-frame, I think a better designed pannier frame could do both jobs. We met othet people on the road on XT600’s without this extra frame support that had no problems with their rear sub frames though.
I also had a large plate fitted to the bottom of the side stand to stop it sinking in sand or mud, very worth while, and didn’t regret not fitting an aftermarket centre stand.
The aluminium pannier can be removed an wedged under the right foot peg bracket (front brake lever tied back) to get the back wheel off the ground to lube the chain / fix punctures etc.
I had a ‘Prop-stick’ that I used hooked under the engine crash bars to get the front wheel up off the ground. This is a product that I bought from M & P in the UK, it’s a 3 piece rod threaded together with a Y section at the top (Like an axle stand) and a base plate at the bottom to stop it sinking into the ground. You wind the middle section round to make it taller. It’s a bit heavy & bulky though.
I also got a screen from Wunderlich, made by MIRA I think, which worked well, and a tank bag that held a map at a good position to glance at while you ride.
Most importantly we threw sheep skin fleeces over the saddles for extra cufrort, they may seem fine for an hour, but they are murder all day without them
Better suspension is also worth considering, especially front fork springs and fork oil change if larger tank fitted, but none of this is essential
Have fun planning and modifying