If you are serious about making big hp and staying under 700cc it can be done. Money ,time, and work are going to be the limiting factors. You need to find someone who is willing to to work with you on custom ground camshafts. You will need to spend an ungodly amount of time on the dyno and track testing each and every combination of parts with different camshafts. A good engine builder can get you a few extra horses out of the bottom end of the motor as well. I don't think there is a limit on how much you can get out of any given motor, the limit is how much you want to put into it. There comes a point when you ask yourself if its really worth it.