hi curchunflo and welcome to the forum. i think the hard part will be getting someone to ship to Argentina. most of the 2 stroke kits are pretty much the same. most will get around 25mph. if you are looking at a 4 stroke consider what motor company's are popular in your country, Honda, Subaru ect. my best advice i can give is email all the dealers you can find and see how much to ship you a kit. i highly suggest you get a kit and put together yourself. reason i say this is that way when you have a problem you know the bike and able to fix yourself. my gas kits were basic and they worked well for me. shift kits and things like that is your personal preference and not a necessity. depends how fast you want to go and what you are trying to accomplish. for starters a basic kit in my opinion would be the best way to start. you can always ad shift kits, custom pipes and all that stuff later if you decide to.
also their is a lot of members from argentina. here is how you find them http://motorbicycling.com/showthread.php?t=17792
maybe one will live near you