You can spend about $1000 on an exhaust and intake to gain a couple of kw, maybe even 3kw if you are lucky. Or get a piggyback to play around with timing and fuel to get about another 5kw, but that's pretty much it without spending serious $$ on cams, aftermarket management, forced induction etc.
Better to drop a 1NZ in instead and then go from there.
By the way, Toyota suck, they want $200 for the tiny little 1NZ water pump.