2,000 16V PSA

I need to know how I could get more power out of a 2000cc 16v PSA engine with 135 hp. It's not bad at all, but if I could get a little more, that would be great! I've been given some ideas: polishing the intake, skimming the cylinder head, changing the ECU and intake... would that be reliable? Thanks in advance!

Most of the work to be done is on the cylinder head; if you cross the camshafts you can get more power, in addition to other things, like drilling pistons, but none of this makes much sense if you don't use the vehicle for racing because the cost is high and fuel consumption increases considerably. You can modify it just with a ECU, but it's also expensive.