Even after the "best" alloy is found, the design of things like valve seat inserts can make a head prone to cracking or even an engine unreliable (dropped valve heads).
An old 'piston' alloy, 242, is a common aircooled cylinder head material (H-Ds and some aircraft). A couple percent Ni seems to be a help in providing the strength at high temps.
Tmoose is right on target. Seats dropping out of the head is your biggest design worry. The distance (think both material and cooling) between the valve seats is something that you must work to obtain. (0.15" between the seats is a ball park estimate for something with around a 3" bore).
EngJW is right on with the comment about 355-T6. One of my colleages was the metallurgist at Continental Aircraft Engines for a while, and he told me that the fatigue resistance of 355 was far superior to 242 alloy.
The 300 series aluminum alloys are primarily aluminum alloyed with silicon. These alloys pour well, and are the bread and butter alloys for most aerospace and automotive use. But they are not the best at high temps, therefore are not the best for air cooled engines. Continental and Lycoming both use a 200 series alloy (aluminum alloyed with copper) for the cylinder heads of their air cooled aircraft engines. Duetz Diesel uses a special alloy called RR350 for the cylinder heads of their air cooled diesels.
RR350 is similar to A201 (A201 is an alloy designation that is more common in NA -- RR350 is a European spec). A201 is relatively expensive (it is aluminum alloyed with copper and silver) and hard to pour (consider only a foundry expereinced with A201). But it is the best in terms of fatigue life at high temperature.
Sounds like RR350 might be similar to U.S. alloy 206, which is basically 201 alloy without the silver. The silver was originally added to enhance the resistance to stress corrosion cracking, but later research and experience has shown that there are a lot of applications where it isn't needed.Thus, 206 costs less than 201. The 201 or 206 alloys would be a good choice, since you are starting out with higher tensile properties than 355 (50ksi min vs 35ksi min). Also the ageing temperature for 206-T7 is 390F, as opposed to 310F for 355-T6, so you can run higher cylinder head temperatures without getting a loss of strength due to over ageing.But, as Turbomotor noted, you need to work with a foundry experienced with these alloys. I worked for a company that cast 201 alloy in wall thicknesses as low as .10" and we had LOTS of problems getting a sound casting in these areas.
