To clarify, you're seeking more than 1 horsepower per cubic inch.
As others have said, one recipe for more power is more airflow, through the carb, intake, heads and exhaust. But without compression, you're being held back. And without increasing the displacement with a stroked crank, again your limited.
Possibly the easiest recipe and most cost effective way to get there is post #4, N.O.S nitrous oxide. You may still need the long-duration cam, and better airflow parts (carburetor, ported heads, headers) and you can get there.
But the nitrous alone may do it. My opinion is the mains, crank and connecting rods can withstand a 100HP kit.