Very nice to see you guys got to Machu Picchu!
About the chain, well... 18.000 kms for a bike loaded 2up+luggage is not really bad, keep in mind that even though you are traveling on paved roads, they are not the best of roads, with lots of potholes some intermittent sections of dirt, mud washed on the asphalt due to rains, water crossings and others that will shorten the chain's life considerably when compared to riding on paved road in first world countries ... That does not mean it does not suck to have a problem with it... I'm sure you will not have a breakdown if you ride to Perito Moreno, as long as the "tight spot" has the correct slack and you refrain from heavy acceleration you can probably still get out a couple hundred kms (maybe a couple thousand even?)... If it gets too loose, you can always cut a couple of links from it to make it last a little longer... Ask around in El Calafate, it is a relatively "large" town and there should be bike mechanics that can help.... Also, I'm not current with prices in Argentina as the last time I was there was 6 years ago, but I think bike parts would be cheaper in Argentina than Chile so ask before you cross the border.
Good luck, I'm sure you'll solve the problem and get back on the road soon enough.
Keep having fun.
SS. '98 BMW F650 / '06 WR250F / '07 KTM 990 Adv