Any free-flow will do. It depends on whether you're going to change your whole exhaust or not. You might want to go too big, though... 1 7/8" might be what's best.

A muffler alone won't do much... you need to change everything to get more power... and even then, it might not be enough. I had a 1.3 once with a full kit, Intake, headers, midpipe and muffler. It was still slow as molasses compared to a 1.6. A GrupoToyota member has gone even further with his, he's gotten cam work done... but all it does is make it a little quicker, but not as quick as a 1.6...

The one good thing about a 1.3? With the lighter weight of the 1.3 model versus most others (1.3s are usually stripped of amenities and have less weight in front (smaller engine block), a well-done 1.3 is nice to fling about on a race-track.

In other words, don't sweat it... it might be best to concentrate your modifications on more useful stuff, like lightweight mags and good tires.