How do you convert engine cc's to horsepower?

There is no direct method, An engines displacement is only one of several factors that influence power output, Some of these are: compression ratio, ignition timing, camshaft, piston and cylinder head design, type of fuel intake system, efficiency of exhaust system, etc, Even when engineers design an engine for a specific power output, it is only proved out by dynamometer testing of the engine,.Even mass-produced versions of the exact same design will vary slightly and change with use and wear.