The MYSide carburator conversion is said to vastly improve throttle response in the 1.3L, as does the MYFI system. The first is a conversion using the excellent Harley Davidson motorcycle carb, while the second uses components of the Volkswagon fuel injection system. Both systems are manufactured by
ZOR.
A good set of exhaust headers, such as the Doug Thorley Tri-Y, coupled via 2" tube to a decent high-flow muffler will help somewhat. Nothing reasonable will give great gains... around here the popular move is to swap out the 1.3L with an 8 valve 1.6L.
Good luck!