Not knowing how far you want to go and how much money you'll spend, there basically 3 ways to go. 1, Turbo- or supercharging, 2, hotter cams and exhaust or 3, optimizing the ECU program.
1. Novitec and maybe others did kits for this, but they are not listed anymore, check
Alfa Tuning, Fiat Tuning, Lancia Tuning, Novitec Tuning für italienische Fahrzeuge
2. Cams, sports air filters and exhaust can still be found, if this is fitted a remapping of the ECU has to be done to get all potential out the engine and it running right.
3. Optimizing the ECU program is the same as above but without modifying the engine. Squada Tuning and Super Chip do preprogrammed chips which go some way improving power output.
You mention the rev range. I used to own a 155 2,0 16v (same engine as yours) and found that it was lacking power in the lower end of the rev range. I had it fitted with a UniChip which is remap able. The engine gained 13% more power (HP) and from 8-13% more torque (most in the 1500-3000 range) which was very satisfying.
But before doing anything, have the condition and tune of the engine checked, no use spending money on tuning a worn engine.
Erik