I'll list only foreign-born people currently representing Argentina in olympic sports, don't want to speak about ancestry of our people because it would be complex to classify, like for most nations in american continent. There could be someone else in a team sport that I'm forgetting.
Matias Albarracin (Equestrian jumping, olympian 2016, but his family is argentinian and he has lived here since he was a kid )
Iryna Khokhlova (modern pentathlon, olympian 2016)
Sol Fainberg (Rhythmic Gymnastics, pan am games 2019, argentinian parents)
Stefano Lucchetti (Fencing, pan am games 2019, argentinian parents)
About argentinians currently representing other nations in olympic sports that will probably be in Tokyo (so, not counting athletes that have already failed to qualify):
Valentin Verga (hockey, olympian 2012, 2016)
Gonzalo Echenique (formerly) & currently (water polo, olympian 2016)
Damian Quintero (Karate kata, as stated above)
Pablo Cuevas (tennis, just born here but lived in Uruguay all his life)
There are also some athletes not born here but with argentinian roots scattered around the world (most notable would be american Diana Taurasi, the basketball player) but I can't remember them all right now.
About the past, there have been numerous argentinians representing other nations, mainly Spain and Italy (obviously). In the case of italy a lot, specially in sports like football, rugby and field hockey, but also some in sports like volleyball or sailing.