There are some good mechanics in Oaxaca (from reports I've read ... never met them or used them), but one guy had his Trans rebuilt on his DR650 in Oaxaca.
Pretty serious job and once done, rider made it to the US on same bike, no trans problems. So good guys out there ... your job is to find someone good who can crack that KLR shock for you.
IIRC (my last KLR was 1998) you can remove the spring without compressing it. (if you know how!) I would read up on the forums or ask questions. This is not hard stuff. (but that's coming)
The harder part is cracking into that Kayaba shock without ruining it. If you search, you will find tutorials on doing this. Probably not U-tube ones, but written out with pics (if your lucky).
With some decent tools you should be able to do this yourself, or at least supervise a local mechanic to make sure he does it right ... NEVER, ever leave them alone. You need to BE THERE. (trust me on this one)
The other thing you'll learn once you start reading up is that your shock will need a Fresh Nitrogen charge. Common Auto shocks need this too ... so any
decent Taller that does suspension work should be able to re-charge it.
DO NOT use too much pressure or OVER charge it as it will BLOW OUT your fresh seals. I believe Kawi lists a spec on this. (owners manual)
I believe 5 wt. oil is standard. I would not go over 7 wt. (mix 5 wt and 10 wt 50/50) Most experts would insist to use 5 wt. and add a heavier spring, but since you can't do that, go with 7 wt.
It will be plenty. It's the Nitrogen that will help the most (controls rebound damping ... very important) The problem is ... unless you modify the oil passages within the shock ... a heavier oil will only screw things up. Get it?
If the shim stack and washers are modified correctly, you may be able to use a heavier oil with good results, but this is a bit tricky so, I'd leave things alone, use 5 wt or maybe 7 wt.
I'm sure your reading and research will cover this if you get onto a good thread on one of the forums. Lots of good info out there!
I assume you've cranked up your pre-load? If it's bottoming out continually, even on mild roads, then I would either get another (heavier) spring or take weight off the bike.
Be sure to inspect and re-grease the linkage bearings when you've got it apart.
Good luck with the ALL BALLS kit, hope it fits and lasts ... a while!
TIP: Guatemala City has lots of Bike shops and KLR's are not unknown.