50 extra hp huh. Like stated above, blitz super charger kit will give you about 30 extra HP and the power enterprise kit will give you about 50 extra hp (assuming a stock yaris is around 90-95whp). I believe those numbers are about right. Then yah, you can add headers, carolla/matrix/xd throttle body, intake, richard's intake manifold (if you dont get the blitz S/C kit), pulleys, a cat back or the nitto midpipe/axleback. Or you can wait for a turbo kit to be built (hah!) or tackle the task yourself. But I heard if you slap a Micro Image sticker on your car, it adds 40hp. Have fun.
* you can always try the ebay turbo kits online. but those won't be good..
http://cgi.ebay.com/ebaymotors/JDMS-...Q5fAccessories