There are a dozen countries in South America (one is not an independent country, but it might as well be for this purpose), and each has its own set of rules, variously subject to change and better or worse enforcement. If you're trying to figure out how to buy a local bike in South America, you'll need to start looking at individual countries--not the continent as a whole. Once you've settled on a couple of likely countries it becomes a lot easier to wade through the morass of applicable regulations and current practices. There's plenty of information on this forum--some of it up to date, some hopelessly outdated. Start reading.

The other approach is to buy a used (non-local) bike from another rider. Again, there is an almost infinite amount of information on this forum, although most is not as detailed because most such transactions have some not-quite-fully-legal aspects. Buying a non-local bike isn't as country-specific, and bikes may be more suited to North American and European tastes. On the other hand, there aren't as many of them available at any given time, and there are some definite legal pitfalls. You'll have to read more carefully, and some of the information you need is hinted between the lines. Nonetheless, it's all here.

Basically, you're facing a large, intricate decision tree; make the big decisions first--What kind of bike? What time of year? How much time to arrange a purchase? How's my risk tolerance?--then gradually focus on smaller decisions one at a time. Eventually, your path forward will (probably) become clear.

Hope that's helpful.

Mark