I've got a friend in Limón with a KLR who knows a bit about the parts and repairs available there and in the capital. PM me an email address if you're interested and I'll put you in touch.
Of course he'd really rather sell his KLR, but he's a nice guy, hospitable and polite (under a crusty surface). Let me know.
Hope that helps.
PS: sounds like you need a new shock, since AFAIK that unit is not repairable. At least, mine wasn't. Not safe to ride. Cost about US$500 for a new one in Guatemala. Steering bearings are an easy fix--either to adjust or to replace. Backfiring can be valves: have you adjusted recently? It can also signify a loose baffle in your exhaust--this, too, happened to my KLR and mystified several mechanics before we figured this out. Of course, carb, plug, air filter and other obvious stuff comes first.