without changing the cam your not going to see a noticable gain for what you are going to spend.for any noticable gain you will need a cam/intake rocker arm combo and you are looking at about 6-700 bucks.for a stock engine like what you have even 1.6 rockers aren't going to give you any gain you...
take and get a pressure tester. you can rent them at most chain parts stores. if you have a leak a pressure tester will find it. you can also get coolant dye and a black light.i would try the pressure tester and dye first.