the thing is, the only way to get big power out of a normally aspirated engine is to increase engine capacity, and considering your sticking with your engine the only thing you can do is work on the existing cyclinders themselves like stroking it, and increasing the rev limit and hope your max power / torque sits above 6.8k rpm. Its just going to be a ball ache and cost a lot money for very little gain. the only way to increase an engines power significantly (bar getting rid of it and using a different engine) is to use some form of forced induction