You can swap to a k20a2 head. The more aggressive cams in say a k24a2 head won't work with the stock a3 pistons.
What you can expect is 170-180 whp with kpro or aem ems.
You can do it yourself if you know the engine. It's not complicated but if you don't build engines i wouldn't tackle it without a helms manual or something similar.
Honestly, better results have been seen with vtec killer setups on an a3. 02-04 rsx-s cams, rbc intake manifold, exhaust,and kpro tuned have netted 190+whp
Price wise, vtec killer is cheaper. An a2 head pricing varies when buying second hand.