Just to put it out there up front (and as you will no doubt find out, as you do a search on the forums), there isn't much you can do with the base Nu 2.0 GDI engine. Sure, you can improve the sound and get MAYBE a handful of horsepower with the usual intake/exhaust/whatever, but for the most part if you want performance you're either doing aftermarket work to an engine designed for efficiency. Or just keep it to suspension mods. There's a few companies that sell a variety of aftermarket parts, such as SXTH Element. There are also, yes, a number of eBay and Amazon sellers that deal in KDM i30 parts, which is really, pretty great.
You can try to source the N-Line parts from various dealers around the US who sell OEM parts online (such as Del Rey Hyundai, in Florida), but not sure what you're luck is going to be. By and large, these parts are slightly modified or just re-painted versions of the N bumpers (ie: silver versus red painted accents). If you're looking FOR the full N parts for the i30, such as the rocker extensions we don't get, I'm sad to say that I and a few other members have looked around but you have to purchase and ship from Europe or Australia... and let me tell you... just the N rocker extensions ALONE, from Oz, are expensive as ****. Try about US$700+... each.
Unfortunately, and especially in the US, the GT was never really meant to be a volume model... and the aftermarket tends to favor the Elantra Sport sedan, and even the Kona, over this car. It's been a source of frustration to even get a gauge pillar mount. Hyundai doesn't yet enjoy the level of aftermarket support that, say, Honda owners have, despite H/K being pretty mainstream for a number of years now.
Otherwise... you already know you should probably trade it in on what you really want (I know, I did the same thing going from a Value Edition sedan, to an N-Line)
