Hello everybody

I'm starting to make plans for my next travel on my good, old and "slightly" underpowered Suzuki GN125 and because of this I'm looking for some help with those who already traveled on other small displacement bikes like mine.

For the last few years I just made some small travels inside Brazil with it and generally (like 99% of the time) in paved roads, but now I'm trying to organize a longer travel to Argentina and Chile and I'm planning to use my current bike for this travel. Because of this I'm on the initial stage of planing the entire rebuilt of the bike and I have some questions regarding the tires I should use.

For the last few years I have been using the stock tire and wheel combo on my bike (3.50-16 in the rear and 2.75-18 in the front, tubed Brazilian made Pirelli tires mounted on the factory original alloy wheels). This configuration is very common for small cruiser bikes in Brazil but every now and then I have some problems to find good quality tires for my bike (generally the rear tire and rear inner tube, the front tire is a lot easier to find good deals).

For this trip I'm planing to go down the Ruta 40 in Argentina and up through the Atacama and I'm pretty sure I will need to replace my tires somewhere during this trip and I'm worried if I will be able to find the rear tire on the way. I could avoid this carrying a replacement set of tires the whole trip but this is an option I'm trying to avoid at this moment so I was thinking, is a good idea to replace the wheels (specially the rear wheel) on my bike for a 18 inches wheels? Now a days Is possible to convert my bike either to 18 inches spoked wheel (like those on the Brazilian made Yamaha YBR) or 18 inches allow wheels (like on the Brazilian market EN 125) and use tires like the 100/90 or 110/90 tires in the rear but um insecure if this is a good option or it justifies the hassle of this conversion. What are your thoughts about this? Should I keep the original size because will be easier than I expect to find 16 inches tires on the way or I should go on and replace the wheels on my bike?