As with any other engine, you can increase the compression ratio by shaving off some material from the heads. After having installed the head back on the engine, start out with a delayed ignition timing, then slowly bring it to the original advanced setting (using a timing light) until you hear it knock. Take the car for a test drive and find out if the engine knocks excessively under load. A little knocking should be fine, especially if you short shift (shift to high gear to soon). Try downshifting when it knocks and see if it still knocks at higher rpm. If it does, try switching to a higher octane fuel or back down the timing a bit and repeat the test.