Not much you can do naturally aspirated to get more than about one HP per cubic inch, and that will be a somewhat rowdy street engine. It all depends on how much lope you can stand and whether or not you need vacuum to run accessories like power brakes. Head milling (about .100") is the first place to start, then more camshaft, bigger valves and port work If you want to go that far. It all depends on "how much" power you're looking for.
