I forgot you were looking for tips to set up your bike for a long tour in Asia, so here is my personal experience.
First, decide if you will leave the bike behind at the end of the trip or rather keep it, as that will impact what kind of upgrades you will make. Then, in this order:
1.- Ride the bike and make all adjustments needed so you can ride for hours on end without discomfort. The saddle post moves freely, the saddle is bearable or you need to swap it, etc. I see the Marin has an ahead headset, which will limit your handlebar height range. I usually like it leveled with the saddle, but your taste might be different. In that case you would need another stem with bigger rise or a riser bar
2.- Completely dissasemble the bike and build it back. You get to know the bike, check for cracks, threads no longer good, seized bottom bracket, etc. Check for wear in the drivetrain, etc. and start your wish list on what needs changing.
3.- Build it back, as it is, and start riding it everywhere. You will get used to the saddle, position, etc and notice what does not work as it should and needs replacement. Otherwise, it's likely that even the lowly Shimano Altus on your bike will take you all the way to the Pacific. And by the way, if you replace stuff, the lower range has chainrings and cassettes in steel, which is a good thing because it will last longer than aluminium.
Touring specific stuff:
-I'd rather get quality racks, like Tubus, and remove them at the end of the trip (if you leave the bike behind) rather than risking it with a cheap aluminium rack that might break on the way and will be hard to replace with similar quality
-If traveling alone you might get away with just rear panniers and a handlebar bar. If you need a front rack, just customise a rear rack you can reuse later.
Here there's some info.
-Next would be wheels. If you and your load are medium-light, and your wheels are double wall and ideally 36 spokes, it might work. If you have cheap single wall rims or you like to carry the kitchen sink, better invest in decent wheels. We got ours for our South America trip in
https://www.kurbelix.de/. Yes, the website is only in German, but they have a huge range, including 26", with some sturdy options. We went for Ryde Andra rims with Deore hubs, roughly 100€ per wheel (that was pre-covid and pre-crazy inflation). The Andra 40 are supposed to be for cargo bikes, so they are really tough. I think wheels are machine built and then manually inspected/reviewed, not manually built like others, but the price and quality is good enough for me.
Regarding bike standards and what will be best for easy replacement along the way, each cycle tourer will have an opinion. Usually whatever you choose, by Murphy's law won't be available when you need it. But in my anecdotal experience 26" is everywhere on the cheap stuff (ie. loaning a wheel from a shepherd in a lost village) but quality stuff in the cities is mostly 29". The 27.5" wheels are just a thing in the western world, in most other countries is nearly non-existent. Most shops will have rim brake spares, but limited choice if using disks. And the most widely used drivetrains are 3x8 or 3x9. In the big capitals of course you will find everything, excluding some very specific stuff like Schwalbe Mondial tires, Magura brake shoes or Rohloff spares...
Let's spread the retrobike love... My own '92 Trek 950 is reserved for the tours nearby.
